I'm fairly certain I've asked folks before and gotten a big "No that
would be too difficult" (i.e., expensive).

How easy or difficult might it be to add an effects loop (after pre,
before power) to guitar-type amps which were never designed with one?
I'm thinking specifically of MusicMan heads which are usually available
for cheap; for me their only drawback is the lack of a loop. Any tips
much appreciated.