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
I cannot imagine any low watt tube amp sufficing for your Dunlavy's.

If the Pass Labs X-350 falls short in any way, then replace it with an amp that does not fall short.

When dropping to 4 ohms at certain frequencies the speaker will simply draw TWICE the current as at 8 ohms. This can of course tax many amps. As importantly, with tube amps that often have a high output impedence, grotesque nonlinearities can result in speaker response, truly changing the presentation, often unpredictably, but more often to plump up the mid bass and upper midrange, softening the top octave. For some speakers and hard rooms this can be beneficial, but it's a real crap shoot. With a tube amp you MUST audition more carefully! If you like the spectral presentation of the Dunlavys in your room now I wouldn't screw around with replacing the Pass. If the uopper bass is anemic and the tweeters too bright you might like the spectral trampolining a tube amp might provide when wrestling with a low impedence load. I remember when I tried an original VAC Avatar on my Parsifal Encores, and was shocked at how they completely ruined the neutrality of this fine speaker. Aleph 2 monos saved the day!