See 2nd top item in rack behind Jerry in photo... Also see here for more info and photos of the rack http://wald-electronics.com/amphistory.html. He started using the trio live about 92-3 and belive settled on a speaker emulation system after experimenting with different mic'd cabs and cab emulation. I beliVe two other guitarists with grateful dead also bought a trio but don't have sources for that.more
have owned this pre-amp for about 15 years. The clean channel is warm, deep and complex with tons of headroom, and I would keep this amp for this channel alone. The mean channel has a very nice vintage drive to it, reminiscent of old Marshalls or overdriven bassman amps. It's got a nice grind but lacks in punch for the more modern heavy driven sounds. The Scream channel is very versatile, and can do anything from SLO100-type distortion to the more heavy mesa-sounds. mid eq knob has a huge sweep, and overall the EQ settings can make a huge difference in the sound with this amp. I've got the chrome front update, which is for looks only, but if I'd ever find another I'd buy that in a second.
Clean through to early hotrodded fender sounds and a wee bit beyond then this is one of best preamps avail.
Clean is like a black face twin with some extra magic. Mean can cover tweed OD to a Marshall crouch and Scream does the early hottrodded fender sound.
Mine even has midi board so my gsp1101 switches the channel for me :)