As of early July, the amps were finished. They make a fine collection. The knobs on the Gold amp need to be swapped out for silver Marshall style knobs, and the Black amp still needs some circuit tweaks (the cathode bias mode is too hot and the cathode follower on the first stage sounds the same no matter which mode it is running in), but they are all playable and have been in use for a couple months now.
Sonic Appraisals:
Black
In standard Deluxe mode, it sounds pretty much as you would expect. The Mesa Boogie style Thick switch (as seen on Lone Star amps) behaves nicely, adding a great deal of thickness to the upper mids and helping the sound cut through the mix. The gain ranges from spanky clean to early Santana-style growl.
In Blue Angel mode, it gets dirty quickly. Even 20-30% gain provides a nice crunch. The one is thick and meaty, but never creates the hissing/sizzling distortion one associates with metal. Rather, the dirt is all overdrive, and all rock-n-roll. The EQ controls, being early in the circuit, play with the character of the distortion, but the lack of post-gain EQ (aside from some careful treble roll-off in the phase inverter) gives this mode a very forward, middy tone that cuts through the mix. I don’t think this mode would be appropriate for playing background rhythm where your job is to blend in and almost get lost in the mix of the band.
Red
The Normal/Matchless mode goes from totally clean to fairly overdriven, but it is not a high-gain sound. The only EQ applied is in the form of the switchable couping cap, allowing you to trim out some bass and get whatever level of tightness or fatness you want. Beyond that, there is nothing to clear out the mids or tweak the trebles, so the sound is pretty mid-heavy. This mode accepts fuzzes, distortions, and boosts nicely: the jagged trebles of these pedals get smoothed out in the amp’s simple overdrive. This is pretty much how Brian May uses his AC-30s (Normal channel + Treble Booster). It’s neat, but I don’t use this mode without enhancing the tone with stomp boxes.
The Top Boost mode sounds exactly like you would expect a Vox to sound: the extra treble gain enhances the harmonic content of your guitar’s signal and gives you a very rich tone, even when playing clean. When the gain is cranked up, you can even get some vintage metal tones out of the amp (especially if paired with a boost of some kind). The EQ is exceptionally powerful compared to Fender and Marshall EQs. With both controls set to 0%, the EQ is totally flat. With both controls turned to 100%, the mids are scooped a whopping -30 dB. With 100% treble and 0% bass, the response is almost a plain high-pass filter encompassing the entire range of the guitar, and vice versa with the controls reversed. Even though there are only two knobs to work with, I find it to be much more sensitive and satisfying to tweak than the Fender/Marshall controls most of us are used to using.
Gold
In ‘60s mode, the tone is fat and round. Rolling back the volume cleans up somewhat, and the tone gets very sparkly and clear with single coils (especially if you have a treble cap on your guitar’s volume control). It responds very nicely to changes in your guitar’s volume and tone controls, giving you excellent control over your sound from the guitar end of things. This mode blends well with distortion and fuzz pedals: its sweetness is enough to smooth out the jagged high end and create a good aggressive sound without being too painful to listen to.
In '70s mode, the tone gets a swift kick in the pants and kicks it up a notch. The Bright gain has brighter highs, more forward upper mids, and thinner lows. The Normal gain has an extra dose of mids. The distortion is noticeably more crisp and aggressive and less sweet and juicy. The extra high-end definition makes this mode require a bit more care and attention, and it pairs better with overdrive and boost pedals than it does with distortion or fuzz, which tend to sound too harsh and shrill.
An interesting note:
My favorite “extra juice” pedal right now is an AnalogMan Prince of Tone clone in overdrive mode. It’s a wonderful pedal for adding a little bit of dirt and definition to your amp’s overdriven tone, especially considering that it’s just a tweaked Bluesbreaker. I keep it set to unity volume, 70% tone, and 60% gain. It adds just enough oomph to take a sweet, round amp overdrive and push it into chunky territory. It enhances your pick attack and improves sustain a bit, which adds some aggression without having much impact on the EQ or character of the amp. If you are the DIY sort and you play a classic '60s style amp, I highly recommend building yourself a GeneralGuitarGadgets Bluesbreaker kit and tweaking the few values necessary to turn it into a POT.