I have done this many times. My old BBE 383 bass preamp was used to good effect as an EQ for the kick drum on my band's first CD (and in several live situations). My Peavey Bass-Fex pre/pro was used for vocal effects on this same CD (it did have some nice reverbs). I think that my Eden Navigator has been used as a preamp for a line in (CD player) on a couple of occassions. I have used a pair of my EA VL-208's for vocal monitors in a practice room (they work tremendously well in this regard). For a last second PA (when our sound man cancelled on us days before a show), I hauled all my EA cabs to the gig and put a stack on each side of the stage. Here is a shot of the stage right stack:
My rig that night was the EA CX-310 you see in the picture (with an Epifani T-112 turned towards the drummer), powered by my iAMP 800. My rack rig (on the floor) was powering the mains. It's very nice having a PLX 3002 in your rack! My old rack rig (next rack up) was powering the monitors. That amp is an Audio Centron RMA-1600. Interestingly, although we ended up using a friend's wedge monitors for on-stage vocal monitors (he showed up with them at the last minute), before he got there, I had been using my three Epi cabs (two T-112's and a T-110UL) to set up the monitor mix, and they killed the wedges. I really should have stuck with them, as the wedges (I think they were Yamaha's) gave me feedback troubles all night, whereas the Epi cabs got darn loud with no feedback.
Tom.