You can go higher but not lower. Say, run an 8 or 16 ohm speaker off a 4 ohm drive. However, running a 4 ohm speaker off an 8 ohm drive is most likely bad and will damage the amps output transformer and possibly eventually take out power tubes and cause other cool things to happen...well...cool if you like fireworks and pyrotechnics. Why not get the right impedance speaker for it?
