hey. *i'm a proud owner of a dual rec. rackmount and it sounds great! *there's no difference between those and the regular heads. *same controls, channel cloning, bold/spongy power, silicon diodes/rectifier tubes, the whole nine yards. *and yes, the bias is set so you can choose between 6L6's and EL34's. *i think the only difference i noticed was that (if my memory serves me correct) the non-rackmount heads have two 8/16
ohm output jacks and two 4 ohm outs. *the rackmount only has one 8/16 ohm output and two 4 ohm outputs. *no big deal at all. *you can check out the specs at
www.mesaboogie.com and you can even e-mail them or call them and ask them about it. *they're really helpful. *i love my dual rec. rackmount and i'm sure you will too!