By April 2nd, Pál Nagy had finished my custom fixed mini filter to bridge the gap until his fully variable Gradient Box would bow. Its measurements show a THD of 0.00072% and noise floor of -130dB. None of our amps match these figures. That filter would play at the front of the I'm-not-really-there line to be sonically invisible.

Next we see the high- and low-pass slopes with the red vertical lines marking the -6dB/40Hz crossing. As to making such a filter available officially, Pál learnt from "feedback of 4Pro SE clients that a fixed-frequency filter is no real solution for them. They don’t yet know what they want. They need to experiment first and fine tune settings. They will wait for the Gradient Box which will offer a lot of filter customization."

With my proven fixed filter, I could now swap the downstairs Vinnie Rossi L2 Signature linestage and Pál's full-function icOn 4Pro SE autoformer passive linestage with the same filter built in upstairs.

Why? Something had happened in the interim: new amps in the big system. Playing the L2's direct-coupled/heated triodes of 900kHz bandwidth into these  2.5MHz direct-coupled monos betrayed inside-the-music noise. That acted like a strange metallic dither or grit between the notes. Apparently the 400kHz i/o filters of the earlier LinnenberG Liszt monos had blocked that noise. I'd never heard it. With no tube output transformers to act as a low-pass filter against UHF noise, the amps' own extreme bandwidth didn't block it. Now it passed clear to the speakers. I could easily kill it by killing the tubes. It's a smart feature of this preamp. It can tide one over whilst waiting on tube replacements; or compare their contributions to a true bypass. Simply toggle two switches and the DHT are out of the signal path. Now the preamp acts as a precision passive with active zero-gain output buffer. The passive bit is like my icOn 4Pro SE, just big multiples costlier so wasteful.

Having acquired the L2 for its tubes, I wanted them back sooner than later. I just needed the right hardware context with a less 'exploded' amplifier again. With the new external filter box, upstairs now was ready for the Vinnie Rossi. Here's how that hardware set up. Once the Dynaudio arrived, it would bed right in. Until then, I'd not use the filter and run the speakers full-range. Options are good. To review, I must be able to bypass any sub and report on a subject's uncut performance, be that a speaker or piece of electronics.