Options. With my icOn 4Pro SE passive-magnetic preamp from Pál Nagy, I had fixed hi/lo-pass outputs at 40Hz. That gave me ideal mirror-imaged analog slopes on the sub/mains and a phase-consistent handover between them. I just had no means to control the SB1's volume. Enter a Kinki Studio THR-1. Its line-level outputs of Exicon lateral Mosfet current buffer follow an Alps pot. That became my sub volume. In the unlikely event that the sub needed more not less signal than the mains, I could cut the latter's signal with the usually bypassed passive analog attenuator inside a Crayon CFA-1-2 speaker amp. Pál's forthcoming separate Gradient Box will add much flexibility to my fixed built-in filter module with his very own discrete digital potentiometers [early samples below of which the final circuit will need 16 units].

The all-analog Gradient Box with zero latency will have 4th-order filter stops at 26, 40, 53, 60, 80, 92, 106, 120, 132, 145, 160, 171, 184 and 198Hz; mono/stereo sub mode; ±10dB gain trim for the sub; continuous phase for the sub; 8-10 different LF EQ settings with bypass; and all adjustments controllable by remote. I'd hoped to have a sample when the Bros. Grimm arrived. The pandemic's knock-on effects in the semi-conductor supply sector had simply thrown a monkey wrench into everyone's schedule. I wouldn't be able to move the sub/mains inflection point up to have the SB1 handle ever more bandwidth, the mains less. With their advanced tech, one might want to let the Dutch sub/s do a bit more than an octave. I was derailed from reporting on sweet-spot tuning with variable filter frequency. That remains for another reviewer, in our 2-channel space perhaps with a JL Audio or Wilson analog xover; or Bel Canto, Linn or TotalDAC digital. To compensate, I'd run the SB1 with sound|kaos Vox 3awf monitors upstairs, Aurai Lieutenant 3-way towers downstairs. But again, a 40Hz 4th-order Linkwitz-Riley crossing denotes a -6dB point. Grimm would work still higher just more steeply rolled off to be -24dB@80Hz.

Here is the low-pass graph…

… and high-pass opposite.

Finally distortion plot, xover board and chassis of my icOn 4Pro SE. Some feedback from Pál's customer Fabio: "The icOn4 Pro arrived safe in Zurich. I hooked it up and everything works. The icOn4 performs very well. Soundstage, details, tone, dynamics and imaging are phenomenal. It well beats all the active preamps I have/had and is on the level of the Einstein [Light in the Dark, tube-based, $10'900 – Ed.]. The Einstein is maybe a tad more holographic while the icOn has slightly more detail and imaging is more precise. The sound is very pure and whatever tube I insert into my LampizatOr TRP is what I get out of the speakers. The icOn4 seems to have no coloration at all. Also amazing are the dynamics at low listening levels which my neighbors will like."

The reason for the latter effect is that like transformers, autoformers transform voltages, currents and impedances between i/o taps. In this multi-tapped case, a 15dB cut of the input signal already creates a 5.62 x increase in output current. 30dB attenuation generates 31.25 times more current, 45dB equals 178.57 x more. It's also why I like to enter my icOn 4Pro SE balanced. Higher source voltages translate to the autoformers. Those can now invoke higher attenuation which delivers more current to the amplifier's input stage. Time to get hooked up.

Just then Pál wrote in to ask whether another fixed lo/hi-pass filter of a new value would help the assignment. Would it? He'd give me the same board as above just with new values in its own small case. I asked for 80Hz so an octave higher. Now I had two filters. I subsequently learnt of Mark Longley's SublimeAcoustic K231 crossover. I bought one with 40, 60 and 80Hz filter cards. Now I felt set to do the subs proper justice. They arrived with Grimm's own 5m XLR cables and generic 3m power cords. This TRH-1 with RCA in, XLR out and Alps pot interfaced between my RCA low-pass outputs and the SB1's XLR inputs without adapters whilst adding vital LF trim. The first theater of operations was my new upstairs sound room with its—inherent in 'upstairs'—suspended flooring. Predictions, anyone?