Ohms is just a measure of resistance. You can plug it into ANY rated ohms speaker- but for best output they should all be matched.
For example, If you had a regular set of stereo speakers rated at 8 ohms, and you connected a second pair it would bring the resistance down to 4 ohms, and the output would seem louder to you (get it? same power through less resistance)
OK thats the best laymans terms I can come up with on short notice
OK. So if I have four speakers @ 15 ohms each are they now 3.75 ohms wired together? With tube amplifiers (the only type that matters) it is integral that impedance is matched. Most amps can be switched between 4, 8 and 16 ohms impedance. Would I want to select 4 ohms (close to the combined impedance) or 16 ohms (close to the individual impedance)?
All I know is that a good tube amp is something of an investment and I don't want to screw it up.