I've been working interactively with a harp player on customising a VHT 12/20 guitar head for harp use. It took quite a few trials but he's been using the amp regularly since the last update.
Here's what was needed for his Chicago style;
1. The treble needed reducing considerably, with the emphasis on bass and lower mids. I had three goes at this and each time the amp came back with the owner saying he needed less treble. It's pretty useless for guitar now, though. The tone stack was replaced with a modified 'Moonlight' stack and the static preset FMV stack bypassed completely. This also gives better drive to the following stage.
2. The input stage was reconfigured to give much earlier break up with a harp mic. This meant lowering the B+, increasing the anode load resistor and re-biasing, more like a Harmony amp configuration. Some experimentation was done to reduce feedback at higher volume, and quite a few NOS preamp tubes were auditioned. There's now very little clean headroom.
3. The standard unbranded Chinese 6V6 output pair were changed for JJ6V6s and this gave a slightly less-harsh sound when the amp was cranked.
This is the non-reverb version of the amp, which is easier to work on.