Actually, I've done a number of 2/5 channel combo set-ups in my system. You can use some of the newer more current tube preamps with "processor bypass" (i.e, Audio research LS16 or Sonic Frontiers LIne 1, or similar), and simply loop the 2 channels through that way. Or, I've also tried what I currently use, wich is a passive/active Pass Labs preamp, and simply put the output of the right/left channel out of the Lexicon into one of the auxilary inputs on the preamp, and do the "12 oclock" thing on the preamp,and adjust the volumes accordingly. REallly, this works just fine! You'll play around with it for a while, and you'll get it, I'm sure.
good luck
good luck