"Clean channel - Green light
This is the clean channel, obviously. It's got a nice, warm sound to it. It doesn't break up unless the strings are hammered into, at this volume anyway, and even then it's very subtle. Can't test it much louder. The sound with the neck pickup is quite "deep", I can pull a pretty spot on "shine on you crazy diamond" tone out of it. On the bridge pickup it can get a little OD, but again nothing unless picked really hard. I owned a Fender Deville 2x12, and currently a Marshall JCM900, I'd say this channel coupled with the reverb and nice pickups (my guitar isn't amazing) that this channel is voiced more like the Fender.
Clean Channel - red light
This is a boosted clean channel. And it's very sensitive to the guitar's controls. You can go from clean, to a Fendery OD (I say Fender, think Hendrix sorta strat tones), to a cranked JMP AC/DC sorta sound (think 'touch too much' and 'Whole Lotta Love' by Zeppelin). This channel really responds differently to different guitars.
Distortion Channel - Green
This is the "British" voiced overdrive, and is the "classic rock" part of the amp, where I imagine you would spend most of your time. It has far more balls than the clean channel OD, sounds great for the heavier AC/DC stuff like 'Whole Lotta Rosie'. Using the volume controls on the guitar you can get a trebley, raw marshall style gain, that sounds great to my ears. With the neck pickup the amp transforms into a very deep OD more voiced like the Deville OD (but nicer, obviously). Two COMPLETELY different tones at the flick of the pickup switch. I set the gain to 4/10 on this channel, so I could compare it against the modern distortion on the other channel. Quite alot of gain on this channel, but even at full it's a very controllable gain. My advice is leave this at low gain and use it as a classic OD as this is where it excels imo.
Distortion Channel - Red
This is voiced more like an american metal amp, but with less gain. The gain is set at full to contrast it to the previous channel. With the eq all central, this channel is very bassy and isn't as responsive with the guitar controls as the rest of the amp. I'm running guitar>amp and the channel is crying to be EQ'd properly, and given a boost, you'd have pretty spot on Metallica sort of tone. Without the boost it'll easily throw out 'Killing in the Name of', think that sorta level of gain, without pedal boosts, with MV at 3. As you move the MV up it get's more gain and more "open sounding" like with any valve amp. I prefer the green channel, personally.
Overall: you can coax nice cleans, mild OD, raw OD crunch, british voiced distortion and american style distortion, and this is with all EQ central.
The amp has a 4 button footswitch to control channel, sub channels and reverb.
A valve change would definitely transform what's a good sounding amp now back to the raw monster it should be. Preamp valves make alot of difference, for a more Fender sorta sound then stick a 12at7 in V1. I put in all tung-sol valves in when I first had it done and it sounded great, I've since put EHX ones back in as I stopped using the amp as my main and wanted the valves for something else. They're mechanically sound, albiet a little noisy and replacements could make a huge difference.
Playing directly into the amp, I found a tone I could use on every channel with no EQing or amp adjustments. It's very diverse just from messing with guitar controls. New valves, as you'd need to do with any amp, will make this a great amp again. "