I made tone stack boxes for the effects loops of the VHT Special 6 Ultra amps Drew and I have.
I love these little amps but the tone-shaping controls unusual and don’t work great with my main guitar (Epi 345). The amp just can’t get bright enough, and it’s a bit thick / congested in the mids with no way to dial that out.
It has a completely fixed FMV (AKA TMB) tone stack (the sort most amps, including classic Fenders and Marshalls, have), with a guitar-style Tone control, a “Depth” which adjusts the frequencies amplified by the power stage from full-range to emphasizing treble, and a “Texture” which offers two levels of something like presence reduction (or no reduction) (it’s not a normal presence circuit, it’s just a capacitor dropping high treble to ground post power transformer).
This box, which gets inserted into the amp’s effects loop, is a low pass filter (bass) and high pass filter (treble) with variable cutoff frequencies in parallel, and a balance/blend control. (So, a super tricked-out Muff tone stack in pedal nerd terms.) The intended use case is dropping a mid notch wherever you want it, plus offering additional bass/treble balancing flexibility, and it does that. But it also does some cool bandpassy/mid-bumped things when you cross the filter cutoffs over each other.
I anticipate mostly using this amp+box for overdubs; record a main guitar part, then dial this in for a complimentary tone to record the overdub. But just by adjusting the box the amp can do a decent Fender-y clean, Marshall-y crunch, Vox-y treble thing, etc., so it could definitely see duty as the main amp on songs that call for those types of sounds.
The general principles in the box should have wide applicability to other amps and pedal designs, but the particular component values I used have been optimized for these specific amps. I don’t have any other amps with loops to test on, but I’m curious how it will sound with other amps.
Bonus: it’s the rare tone stack that is actually designed to sound neutral with “all knobs at noon” It’s not completely flat; there’s a bit less mid/low mid, addressing exactly what I like least about the VHT and presenting its basic tone in a slightly more flattering light to my… ears. Eyes. Whatever. (Leave the mixed metaphors to the professionals, kids.)