many of the PA heads have baxendall tonestacks which, imo, makes for a wierd sounding guitar amp. they generally have alot of negative feedback for max headroom, but thats easy to change.
so long as the amp is PTP wired, the scope to easily mod is huge.
my next project (once I'm out of hospital) is to rebuild my Epi VJ with a Marshall 2203 preamp, all PTP wired running any output valve (single ended) from a 6V6 to a KT88 - should be a little beast.