Sacrilegious? I'd hardly think so. When the MP-1 came out, I'd venture a guess that it was probably run into a JCM 800 more than any other amp head at the time.
Use it with whichever amp head you'd like; the preferred method is to put it into the amp at the power stage and bypassing the amp head's preamp section as previously stated. Nothing wrong with going straight into the front Input as well, but be prepared for the extra noise, and matching proper levels going in and such.