More thoughts on the reverb subsystem. I think it will need some kind of tone control. A bright reverb will sound more responsive and immediately gratifying, but many real acoustic spaces have a high frequency rolloff characteristic; thus, a darker reverb, although it may not call as much attention to itself, may ultimately prove to be more realistic-sounding and more supportive to a good guitar tone.
So at the least, a control which can reduce the treble frequencies seems important. However, since the reverb already requires tubes for the drive and returns, the possibility appears to be available, to make the tone control active: i.e., able to boost treble as well as cut, with "flat" reliably centered in the middle. This amounts to half of a Baxandall tone circuit, just the treble knob, with bass effectively hard-wired to "flat".
But, as long as we are adding facilities to flexibly alter the tone of the reverb in useful ways, within the space of one knob, we can make more options available by using a dual-gang pot with a pull-switch. When pushed in, the control acts as a simple Baxandall treble knob, as described above. When pulled out, the two gangs of the pot become Baxandall treble and bass controls, both moving in parallel: thus, when rotated clockwise, the control boosts both the bass and the treble, creating a response curve akin to the "loudness" curve on a home stereo, i.e., a curve which is mildly mid-scooped. Rotating the control the other way, counter-clockwise past the 12:00 "flat" position, the bass and treble are cut while the midrange stays the same, resulting effectively in a mid-boost curve. This pull-switch functionality is called "pull contour". I have not decided whether to label the overall control "treble", or "tone".
So, the reverb module now contains four knobs: the six-position source selector switch, the pull-contour tone control discussed here, and the two wet/dry mix knobs for left and right.
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