If you have a combo amp with an output for a cab with a cab hooked up to it would both the in-amp speaker and the cab's speaker be in use or just the cab's?
depends on the amp... but I think most disable the internal speaker when sending out to a cab.
Depends, most of the time the internal speakers on the amp turn off. My 5150's speakers don't however, but I'm pretty sure they normally turn off, so mine's been modded to not turn off when plugged into a cab.
The speakers in my Crate combo keep working. In my Valveking the internal speaker is cut out.