If there was one thing I wish every player knew about their amps it would be this:
The volume knob position means absolutely nothing.
"5" does not mean 1/2 power !
+1
Your car has a gas pedal. If you put it to the floor and hold it there eventually you'll blow the engine. User discression is needed with cars the same as with amps, combo or not. The only point I see as valid here is that the OP's amp doesn't have a line out for another set of speakers - that's a poor design by Fender. Then again this is about marketing and if they limited the output wattage to the speaker load they'd never have a Rumble line-up over 50 to 100 watts.
Hartke says that you MUST BE CAREFULL with the 30 or 50 HZ control on their graphic EQ, cos' it can results in severe damage for speakers.