Guitar/Bass amps pretty much interchangeable in most cases as far as the amplification process goes (except for reverb effects which should be disabled from the circuit in a high volume situation since loud bass notes may cause the reverb springs to vibrate and generate a rumble/crashing sound). Few amps are EQ'd specifically for one or the other, but either way it's just a matter of changing a cap in the preamp circuit (usually guitar amps roll-off more low freq signal). Almost any vintage Kustom, Acoustic, Peavey or Sunn will do. Just get a 100 WRMS (or more) if your dealing with a drummer! But the Sunn Coliseum 880 is probably the best amp ever designed for such a task.
It's the speakers that matter the most since guitar speakers usually won't handle the onslaught of a cranked bass amp signal. A good 4x10 or 2x12 sealed (i.e. no ports/slots) bass cabinet would be the best compromise sound. Ports/slots will add more low freq range (which may or may not be desirable).
