Can a low resistance speaker be efficient?

I have a pair of Dunlavy 4A's which are 91db efficient and have a nominal impedance of 8 ohms. Ive read however that they can drop to 4 ohms(which I dont really understand). If the speaker resistance drops, would the efficiency drop as well? So are the speakers really efficient? The only reason Im asking is because im thinking about adding a tube amp into the system. I love my X-350 but thought it might be fun to have a tube amp also for certain music.(Does anybody else do this?) Anyway, will a low watt tube amp suffice here or should I look for something more powerful. Thanks