Most always.
Some amps fare better with a loop than others.
But I do not see why you could not separate the preamp and power amp of just about any guitar amp.
good luck
What exactly is it that you are trying to accomplish.
Speaker(s), pick-up, string, intonation, changes of those things are the way to go if you want to change the Tone/Feel of your amp...not adding an effects loop that might have some degrading pull of the preamp.
Others have already asked, but...
.....What amp do you have, and what are you trying to do.?
Just want to take a bell PA already converted for guitar use and use it for tube power for preamps I have. It's a single ended 6L6 (that one has some RCA jacks that i think already act as a loop). Might do it with a bogen chb35a too which has more oomph. Not me, my tech.