Most heads based on a vintage design can be modded for effects loops
and extra gain stages and different tone setups and whatnot. Especially
the Marshalls. For years there has been a market for Marshall mods.
But honestly, if all you want is a good power amp section, look into a
dedicated tube power amp like the stuff from Mesa Boogie, or even
Marshall made (and probably still makes) some.
So yeah, what you are looking for is possible, but unless you like
experimenting yourself and doing work yourself, it's probably going to
be expensive, void all sorts of warranties, and you might not be happy
with what you have.
Try to keep things simple. Most of the best setups are the simplest.