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Basics: With subwoofers, higher distortion means more perceived output. For pimple-faced adolescents, more distortion is mo betta. For more mature listeners of course, this reverses. They don't want to know there's a subwoofer in the crib. No chili pepper machismo to kill off subtle flavors. Equally verboten is interference with the upper ranges. There a sub might inject fat and blur, the former from undue overlap or leakage, the latter from phase errors. The electrically cleanest seam involves bidirectional filtering. The main signal enters the subwoofer via RCA interconnect (in), the subwoofer's filter subtracts the bass information and sends the remainder back to your main amp via a second interconnect (out). This creates a proper mirror image or hand over between monitor and subwoofer. The give and take is balanced. Without bass in the signal, filtered monitors also open up dynamically. Their mid/woofer can concentrate on the vocal range and upper bass to work mostly as a dedicated midrange driver.

The chief argument against in/out filter mode and for augmentation mode instead are the—real or imagined—effects of the subwoofer's high-pass filter since its insertion puts two crossovers in series (that of the sub first, then the monitor's own). Proponents of augmentation mode run their mains unfiltered. They merely 'sneak in' the subwoofer below. While this does keep the monitor signal pure and unfiltered, the splice won't be as mathematically even as in filter mode. The sub's roll-in slope won't match the main speaker's roll-off slope. Here the listener must determine the lesser evil. Mismatched slopes or the sub's high-pass filter? Theoretical purity or improved dynamics from the monitors?

There are more decisions to make. Regardless of filter or augmentation mode, the low-level connection is generally preferred. It avoids coupling two amplifiers, main and sub*. But you need pre outs to run the low-level connection. With a pre/power amp combo, that's a given. If your preamp has one output only, use a Y adaptor. Most better integrated amplifiers have a pre out as well. Some however do not. Or, your preamp may be so far removed from your amp that you own just one very long interconnect, not two. For these and other scenarios, subwoofers offer speaker-level connections. Not all of them add speaker-level outputs but the Amphion does. With the Impact 400, you can go in and out via RCA, banana or spades. The choice is yours.

Augmentation mode with high-level connection

* Bob Carver once offered a so-called Z-Coupler based on an impedance-matching transformer to allow connection of a low-power tube amp to a high-power transistor amp. Subwoofers with high-level connections employ their own Z-Couplers to avoid loading down the main amp which isn't supposed to 'see' the sub. This Z-Coupler is extra circuitry which the line-level connection avoids to generally be preferred.

Filter mode with low-level connection

Beyond deciding on the connection format and whether to filter the mains or not, there's the question of where to put the subwoofer. The higher the low-pass filter is set (small bass-shy monitors will need to be met sooner), the more easily the sub is localized. For that reason and to create equal path lengths to minimize phase errors, the ideal location could be right between the main speakers - unless that happens to be a bass suck-out spot. Good location scouting runs the sub without the mains while evaluating possible positions, perhaps even placing the sub temporarily into the listening chair while checking bass linearity in all possible final sub locations.

Augmentation mode with low-level connection

There's clearly more to happy subbing than a big box with a burly woofer. For music purposes, the quality of the crossover network is vital. How low can the sub be rolled in? How steep is its slope on top where it must meet the main speaker? Ideally, one wants built-in EQ facilities to compensate for the three major room modes below 200Hz. State-of-the-art subwoofers run DSP but the Impact 400's sticker doesn't reach far enough. So much for basics.

The musically most critical thing to nail is the 80 - 160Hz upper bass band right where most sub/sat systems will hand over. Here too much amplitude sounds ponderous and heavy, even dirty. Too little becomes lean and not sufficiently exciting. After installing a new sub on bass-shy mains, there's predictable enthusiasm with the enhancements in the lowest two octaves. Chances are good that first adjustments on sub volume and crossover point overshoot. Over time, one often tones those settings down. A good rule of thumb is that if solo vocals sound worse with the sub than without, something still needs work. Only when they sound more spacious and realistic are you done.

On the Helium 510s and with the FirstWatt F5, I obtained best results in filter mode, optional port plugs installed to turn the monitor's bass alignment into quasi sealed. The Impact 400's fixed high-pass is a 4th-order slope at 80Hz which steepens to 5th-order at the 150Hz rejection band. This minimizes leakage into the higher bands over for example Gallo's TR-3, easy to hear with just the sub running. Setting the Amphion's low-pass to ±80Hz creates the smoothest transition in filter mode. Because my F5's gain and input sensitivity diverged heavily from Amphion's sub amp, I had very little range on the Impact's pot before the bass got too loud. Still, on most of my bass-heavy electronica and organic ambient fare like the five Tulku installments above played back at happy (sub rave) levels, I favored filter mode. It gave me the most even midbass transition.

With all-acoustic audiophile designer fare recorded well—ECM, m.a., Al Sur, enja et al—and played back at lower evening levels, I routinely felt that augmentation mode was superior. Vocals seemed more direct and tangible. This suggested minor losses of the >80Hz signal passing first through the sub's own filter. As always in hifi in at least the financially not yet liberated realms, it's about choosing your compromises. Your mileage will undoubtedly vary. The important thing is to have choices.

With my iDecco and WAV-loaded iPod, I ran augmentation mode at about 65Hz again with the port plugs installed. This little €3.500 system (closer to €4K with stands and cables) was very compelling on all but symphonic mayhem. There you'd want at least proper three-ways for stereo bass into the second octave. As a former musician with orchestra experience, I find large-scale symphonic works the least persuasive on hifis however. I thus don't much do big classical and wasn't in the least perturbed.

The addition of the Impact 400 quite transformed this setup. Imaging got more tacit, tone colors deepened and the reveal of formerly only hinted-at bass created a proper foundation to anchor rather than just float the sounds. Just as vital, the addition of the augmentation-mode sub was pure give, no take. The illusion of realism was simply higher without screwing with subjective speed and clarity. In short, I found a balance a tad on the lean but defined side which best suited my purposes. Those content with mini monitors solo don't know what they're missing. True, they could argue "integration troubles" and be factual. With a sub of Amphion's caliber—an affordable music rather than cinema sub for medium-size rooms—I'd simply not talk of troubles. I'd talk of adjustments which anyone should readily nail over a week of progressive fine-tuning.

Proper subwoofer integration is best described by what it doesn't do. The expected no-shows are: attract attention to yourself; produce lumps; cause thickness or outright boom; take away from vocal immediacy. At civilized levels in my size room, the Impact 400 complied with these basic requirements. Its transformer didn't hum either which can be quite a common demerit.

On the want list are: richer timbres; a deeper more holographic soundstage with more ambient data; more precise imaging; more force on impacts; a larger scale on crescendos and peaks; and 'connective tissue' beneath melodies that suddenly make more sense when their accompaniment is fully audible. Here too the Amphion complied.

Frankly, assessing ultimate subwoofer mettle is beyond my purview. I haven't reviewed enough samples to feel confident knowing who makes the best - in this or any other price class. I'll simply say that the downfiring smallest Amphion is somewhat softer on transient impact than the front-firing Gallo TR-3. This makes it arguably easier to hide acoustically but not quite as impactful or crunchy. It's physically attractive particularly in the dispatched white; and over the TR-3 an even superior blender. While THX aficionados would want bigger wooferage—and with that, DSP equalization—music listeners with two-way monitors in search of higher add-on realism to not sell anything they already have... well, they ought to put this Amphion on their shopping list.

One proof of the pudding is that as monitors go, the Finnish Helium 510s are a rather high-caliber proposition. You'd not want to blemish them with primitive bass of any kind. Neither did Amphion though. It's not surprising that this combo from the same maker should work well. The bigger surprise are actually the very plain but clean attractive cosmetics. They make it that much more likely to actually get away with planting a high-performance subwoofer into the fiercely guarded living room. Chalk one up for intelligent Scandinavian design all around. Finnesse is the word!
Quality of packing: Very good.
Quality of manual: Very good.
Fit 'n' finish: Excellent.
Value for money: High.
Application conditions: Music-first subwoofer that elevates finesse over brute force. Fine crossover adjustments assure proper blending with a variety of monitor speakers.

Amphion website