Can you put preamp in effects loop?
Some outboard pre's can be used like an effect in the loop, but then you have two complete preamps/EQ's/etc. in series, which can get noisy and may distort at your "usual" settings.
Can you use an amp head with a combo amp?
Can You Use an Amp Head With a Combo Amp? In short, yes. Combo amps have a speaker cabinet built-in, but many manufactures leave the connection visible on the back panel of the amp. This allows you to disconnect the speaker from the built-in amplifier and connect your own if you wish.