Why¹. For best timing without delaying the mains with a digital filter, I now want the sub right between the mains so equidistant to the seat. I can't do that with the Zu. So I Iooked at compact subs like B&O's Beolab 19, B&W's PV1D, KEF's KC62/KF92, Paradigm's Seismic 110 and SVS's 3000 Micro. The Dynaudio had my ideal feature set/specs. I wanted two opposed active woofers, not one passive radiator like REL or Sonus faber, definitely no ports. I wanted purpose-designed woofers from the same firm, not off-the-shelf fare. I wanted 20Hz/90dB peaks for pure music not movies in a 4x6m room. For reviews like the pending spl Crossover, I wanted i/o on RCA and XLR. I wanted PEQ to notch out a room mode. I wanted white. When I'd narrowed my search to the S6 then learnt of the 18S, I couldn't warrant the surcharge for a color difference. Black it'd be. Running both mains and sub off a fixed external filter, I'd bypass the sub's own variable filters.

You can see why I wouldn't want that 1.3m tall subwoofer sitting right between these speakers. For that a front corner is the only cosmetically acceptable option which incurs an obvious time delay.

A word on augmentation mode. It runs the mains unfiltered. To successfully mirror the sub's entry to the main's exit, one wants the latter's -6dB figure. It's how a Linkwitz-Riley filter defines. The industry standard publishes a speaker's -3dB point. A ported box rolls off at 24dB/oct. below self resonance. Multiply its published -3dB figure x 0.7 for the approximate -6dB value. A sealed box rolls off at 12dB/oct. Multiple x 0.6 and select the sub's 2nd-order not 4th-order low pass slope. Our ported monitors are -3dB/36Hz. That makes their -6dB figure ~25.2Hz. If a sub's lowest option for its adjustable 4th-order low pass won't go that low but maxes out at, say 50Hz, it'd give these excess output. For a 50Hz low pass, the speaker's -3dB figure should be 70Hz. That's a full octave higher than ours! Now you see why many subwoofer augmentation attempts don't work ideally. Often the main speakers are too extended for what the sub's low-pass filter can properly accommodate. Hello overdone bass.

If you filter sub and mains to end up with mirror-imaged lo/hi-pass sections, the problem of excess overlap goes away. So does the issue of imperfect phase at the crossover point. Using the lo+hi-pass route, you decide where to optimally set the hinge frequency as long as your subwoofer is sufficiently flexible. Many aren't. Also, the standing proviso is the high-pass filter's quality. The signal arriving at your mains runs through it. It's why I went after a superior fixed purely analog filter. It bypasses what's built onto a plate amp so redundant A/D⇒D/A conversion plus DSD on the sub's analog outputs. For reference, external analog xovers from JL Audio, spl and Wilson with all the necessary adjustments executed at top quality charge from €3'000 to €4'500 – up to x 3 what today's entire sub costs.

Why². Why would I want to loop in a sub if our monitors are -6dB/25Hz already? How much music signal do I miss? Four reasons. First, I love certain ambient and electronic music. That often contains synth bass down to 25Hz. Hearing that at full power not barely hinted at makes a significant difference. Two, the quality of actively driven bass generated by specialty woofers with mega power is different than what our passive monitor can do. Three, filtering them with a precision 4th-order 40Hz active LR filter (-6dB/40Hz, -24dB/20Hz) increases their dynamic range. Low bass beyond their reach no longer presents itself at their woofers' voice coils to heat them up. That uselessly drives up their impedance in an inverse square function to compress their response up to 200Hz. (If these were standard 2-ways, their woofer would turn into a mid/woofer to spread this compression effect all the way up to ~2'000Hz where it meets the tweeter.) With the filter, the most demanding frequencies reroute to mega class D with extreme damping factor. That's what one wants for that bandwidth not necessarily the rest. Four, adjustable bass volume independent from the mains conforms to room, mood and personal taste. To sum up, the sub adds bass quality, reach, power and flexibility to goose it for certain fare or run it linear for other music. One can even account for decreasing sensitivity to low bass with falling volumes—the Fletcher Munson curve—by compensating for whisper sessions. In this league, none of that's true for passive bass. All of that the Submission tower from Utah had long since proven. Now I wanted to refine that recipe in the time domain. The fact that the 18S mimics the opposing woofers of the sound|kaos Vox 3awf seemed like a bonus. Hey, audiophiles get their kicks where they can.