… than Pál followed up with a new declaration of independence. I'd sent him this link to my KIH #81 feature. A reader had sent me that. A day later Pál responded. "You just created a new gadget for this improved audiophile listening mode of which Richard Vandersteen but not Paul McGowan approves. As I see it, key to it is the best possible digitally programmed high pass with a friendly price. A sub's own adjustable low pass will do for a basic solution. In the next 2-3 weeks, I plan to design and make the following prototype for you to test. Think small icOn case of 120x120x66mm with the familiar 5V 2.1mm power port. It'll have four pairs of RCA: one input, one unfiltered sub output, one variable-frequency high pass to the main amp. The fourth RCA will be for options like a matching subwoofer low pass. The 2-digit amber display of the small icOn 4Pro will show actual filter frequency with a one-touch button for manual settings and this cheap Apple-compatible plastic remote. Anyone so inclined can upgrade to an Apple wand for a bit of extra cash.

"The Linkwitz-Riley high pass will switch between 12/24dB/oct. slope and have 16 corner frequencies between 40-190Hz. This range could be a different but would limit to 16 stops. The filter will be the one in your icOn 4Pro Model 3 SE. Frequency control will execute with 0.1% precision resistors switched by analog field-effect transistors. This should offer better sound quality and far lower distortion than the digitally controlled DAC-based variable filters on most plate amps. With this I can't make hundreds of corner frequencies but 16 should do. As one option, there'll be a matching low pass for the sub. For listening tests, the output to the power amp can be switched between full range and high-passed."

Though it will likely feature in the icOn Ultimate's option menu and started life during my ongoing dialogue with Pál, this external filter warrants thus here gets its own review. As a separate device, it can bolt unique features onto existing preamps, thus appeal to owners not looking to replace their current linestage. "Any cheap variable filter obviously has quality limits. Low-resolution noisy DSP, state-variable filters or active op-amp filters with pots simply don't make for the best filters. The SE crossover in your icOn isn't fancy but executed with high-quality parts. Its drawback is the fixed nature of its filter if you want variability. Hence my idea to apply the icOn's tried 'n' tested analog switches not to my usual autoformer taps but resistors. When optioned with the low pass, the corner frequencies either lock to the high pass or separate to offer extra bass adjustments. I love to create something new even if it's not my own idea. This one will simply be my execution of Srajan's box."

A gradient box of another era.

What would Pál call it? The Gradient Box? The Shelver? The RemotoCross? "I like The Gradient Box." To summarize this problem solver, it slaps down idealistic if doomed notions to perfectly match a speaker's natural roll-off with a subwoofer's built-in low pass and perfect phase continuity at the xover frequency. It proposes instead to high-pass the mains at the same slope/frequency as the sub to achieve a mirror-imaged phase-consistent handover. Optionally it proposes to upgrade the sub's plate amp filter with its own superior execution. Lastly it makes the corner frequency selectable from the listening seat, then confirms it by numeric display. When low/high-pass filters are locked, the wand simply raises/lowers the inflection point in 10Hz steps. Remote in hand, our ears decide at which frequency we gain maximum benefits of dynamic range, best mid to upper-bass linearity and whatever other sonic aspects shift. When we unlock the low/high-pass settings, each may be adjusted individually. What's more, we have the option of 2nd or 4th-order slopes for shallower or steeper transitions, thus broader or narrower gradients to accommodate ported and sealed speakers.  Added up, that's a lot of flexibility with total convenience. On paper, the Gradient Box specs looked set to make the grade especially when, strange but true, the €3'000 JL Audio, €3'249 SPL and $4'500 Wilson Audio crossovers all lack remote control.

Being pushy, I naturally wanted still more. "Many box subs like our Zu offer selectable bass boost. RiPol/dipole variants rely on it to compensate for their 6dB/oct. roll-off due to out-of-phase cancellation. Could the Gradient Box include a selectable linear +3dB/6dB shelf between 80Hz and 20Hz?" Given the remote's limited buttons, I expected this to be gluttony. Asking just never hurts. If it was too much, one wouldn't use Pál's low pass. By not defeating the sub's own, its other EQ facilities won't bypass. Naturally bass boost consumes extra power. Subs with that option run spare power and a driver expected to stroke harder. Subs without the option must be tested to not be driven into distortion. That's why any bass boost always remains defeatable.