Dynamic renaissance. Here we see the adjusted downstairs. The big Zu sat adjacent to the right speaker. Ideally any sub there would be half the height or less. No matter, its benefit was instantly audible. And it wasn't bass. After all, these 8" two-way TQTW towers do -3dB/33Hz without help. They miss little meaningful bandwidth. Subtracting low bass from these Accuton voice coils did open up their dynamic response well into the upper midrange. Here I'd not done this before, just augmentation with undue overlap. Then the Zu had placed centrally against the front wall. It subsequently just sat awaiting its day. It's when I'd acquired the icOn 4Pro  with custom filter that the Zu rejoined the signal path. It then simply hoofed upstairs to bolt more bandwidth onto smaller monitors; and because the tube preamp without filter provision then hogged the downstairs spot.

This was my first time doing it downstairs and with the sub located as time-aligned as layout allowed. If I'd harbored secret doubts over its efficacy—our subconscious can worry about all sorts of things, no rhyme or reason needed—they were dispelled by the first music track. Just as the Zu had done for the 3-way monitors but even more so now, dynamic range of the mains was the most immediate benefactor because I now had two-ways. Their mid/woofers are busy up to the tweeter to significantly broaden the benefit. That entire range had grown microdynamic fast-twitch muscle. That's why one wants a quality sub with properly high-passed mains. It unburdens the most vital musical bandwidth to behave demonstrably more free and gushing. So there's that: one small paragraph for a review, one large step for my listening pleasure. If I could behead the sub to 1/3rd its current height, my design sense would be fully pleased as well. The Vox D15 would. Baby steps. Westport wasn't built in one day.

Here is upstairs with Pál's external filter next to the pre. By April 13th, "I just ordered today the PCB of the main building block of the Gradient Box, the discrete digital potentiometer. 6 x small DDP modules will go into one Gradient Box sitting atop the motherboard with its fixed components for the active filters, the micro controller and icOn display for the smart features and remote control."

Subwoofer position compensates for its 2.5ms DSP latency which works out to 86cm.

Meanwhile my Swiss friend Dan had taken delivery of his own icOn 4Pro SE. He'd ordered his with the fixed filters set to 50Hz for Cube Audio 10" Nenuphar with dual Voxativ RiPol subs. He reported the same unexpected transformation to broadband dynamics, clarity and micro resolution. Having recommended this icOn version to him, that was good for our friendship. "A 10% improvement" he punned on the phone to indicate that the sonic change was far from insignificant.

Just so, the present number of audiophiles who'd even consider the notion of filtering their costly loudspeakers will be exceptionally low. Top-quality external crossovers are rare and expensive.

Also, the audiophile press at large doesn't promote the inclusion of subs for serious stereo systems. Where it does on occasion, the focus seems to be on REL-style high-level augmentation mode. Sadly that can't do a thing to dynamically liberate the main speakers; nor lower their distortion from reduced excursion requirements.

Today then is all about a path less traveled. To shed further light, I signed up to review Grimm Audio's new SB1 subs with advanced motional acceleration feedback and ultra-low 38µs latency. Because these Dutch subwoofers lack all filter provisions, they rely on a Pál-type solution; or a pre/pro's or receiver's LFE menu from the home-theater sector.

By late April, Pál had his first batch of DDP boards. "My new discrete digital potentiometer is a potential game changer for remote-controlled smart filters, xovers and other audiophiles products. DSP-type features done fully analog with ultra-low noise, ultra-low distortion and no A/D ⇒ D/A or any related latency of course."

By May 16th, Dawid Grzyb had published his review of a fully balanced icOn 4Pro version. It was his first encounter with a passive preamp. He concludes that "as far as sheer sonic excellence, maturity and completeness go, it sits as high as my reference linestage. That now sells for roughly £17'000 before VAT. It's the best one I know. So if you're in the market for a preamp and don't intend to purposely inject chunky syrupy flavor into your system, auditioning an icOn 4Pro before anything else is my best advice." Elsewhere he calls it "the Bakoon AMP-13R of preamplifiers". That characterization never occurred to me but now that it's out there, I wholeheartedly concur. Well put!

As to whether one should, like Dawid did, worry about the supplied wall wart which powers the display and Arduino MCU for remote control, "the linear power supply is one of the variants of concern of the subjective audiophile virus which has infected a big part of the well-informed audiophile community. Because 50-20 years ago and occasionally still today, the old-school linear power supplies had some advantages, the infected guys without any real knowledge, investigation or experience with this matter believe that all SMPS are shit and the expensive analogue PSU the holy grail. So several icOn customers have immediately replaced my original wall wart, my HK dealer offers the Model 4 with an expensive Plixir supply and so forth. I spent several 100 hours on just different power supply possibilities out of the more than 10'000 hours of icOn development and production. I tried batteries in lead acid, lithium-ion, alkaline, Ni-Cd, zinc and more, dozens of different ultra low-noise linear supplies and several dozen more smps measuring them all with an FFT analyzer and listening to them in blind A/B." Pál then explained the details of his isolation solution which I'm not at liberty to divulge; though if you look closely at his PCB, you'll likely spot it. Suffice to say that with the wall wart only powering control circuitry that's strategically isolated from the signal path, I'll personally continue not to worry about it with my icOn kit. Others will follow their own preferences and perceptions. Whatever makes you happy wins. There's no more to it than that. Just don't use a 12V supply like a potential customer did on Dawid's traveling loaner only to destroy it.