no real advantages or bene's to either but choice of one dictates amp choices as a general rule of thumb..if you like Classical music and listen at low volumes a SET tube amp is strongly recommended and you'll need to partner with a high efficiency speaker design >90db and impedance flat at 8 ohms...this is a lifelike combo IMHO.
Also would be helpful to know room size, type of tube amp, and type of classical.
The benefit to higher efficiency speakers is that less amplifier power is needed to achieve a given loudness. The benefit of a low efficiency speaker is that you may be able to operate the volume control higher in its range where it has less effect on the sound quality.
Some speakers are grossly inefficient but sound amazing (like Maggies or some Thiels). Those are cases where even low volumes require a minimum of 10-50 watts just to "get them going". Personally, I'd want hundreds of watts with those two but, on the other hand, I'd have a hard time adjusting the volume on a speaker rated at 102db like coincidents or klipshorns.
Find either an amp or speaker you really want, then match it with what you need.
Assuming both speakers have the same size and type of box, the lower efficiency speaker will have deeper bass. High efficiency and deep bass at the same time requires a large box (or a creative marketing department!).
Now if box size isn't an issue, the advantage offered by the high efficiency speaker is typically better dynamic contrast and a greater sense of liveliness. High quality high efficiency speakers often sound better at low volume levels and often have a more tube-friendly impedance curve (apart from needing less power, which in and of itself is a tube-friendly characteristic) - but there are exceptions.
There are other generalizations that could be made, but on the other hand there are exceptions to just about every rule (except for the tradeoff relationship between bass extension, box size and efficiency).
Duke above did a great job of explaining the factors of a speaker of lower effiency vs. one with a higher one. One other factor to consider is the nature of the load, i.e. the impedance vs. frequency curve. If you have a relatively high voltage sensitivity but a difficult load (one with low impedance and/or high phase angles), an amplifier with a high output impedance could have as much trouble driving that speaker as it could driving a speaker with a lower voltage sensitivity and a higher impedance load. Also, in some cases, your amp will drive the load to satisfying sound levels but its interaction with the load can significantly alter the tonal balance of the speaker.
With your preference for classical music, I would recommend going the higher effeciency route and since you don't listen that loud, try a SET amp out sometime. Good luck.
Agree with Duke's excellent assessment but I might add that one might not need super efficient, "big box" speakers in this scenario. A speaker with efficiency in the 90 to 95 range would work quite well with low powered tube amps and have good, appropriate bass. A couple speakers that come to mind are Coincident, Reference 3a, Meadowlark, Merlin and Living Voice. All are average size speakers.
I do not think it is possible to generalize. I have heard a few high efficiency speakers: Lowther, Tannoy, Avantgarde, and Zu. And I have heard a few low efficiency speakers as well.
In general, low efficiency speakers have a brutal kind of sound. These are typically very linear sounding, with ample bass. The sound from these speakers are typically very impressive. But as we all know, impressive does not necessarily mean they are musical.
The high efficiency speakers I have heard, without exception, suffered from problems. The Tannoys have a lazy coloration that kills the rhythm of music. The Zu's sound distant and uninvolving. The Avantgardes have horn coloration. The Lowthers are beautiful, but lack treble and bass extension. (Yes, I know this is a gross generalization but it is based on the systems I have heard).
Of all the high efficiency speakers I have heard, the Lowthers impressed me the most. But that is my particular musical taste (classical). YMMV.
To echo Duke's take on dynamic contrast. I used to own lower efficiency speakers (Sonus Faber Extrema, Dynaudio Confidence 5), despite their fine sound quality, it takes mega watts to create the jump factor that mimics live music. I later moved to higher efficiency speakers, not 95db but something real world in the 90+ range. Now I can run tube amps which sounds better to my ears, more lively, more micro dynamic, more enjoyable.
I was seriously considering MBL speakers for some times, but despite the huge soundstage it creates and superb imaging, it lacks the high efficiency speaker jump factor and I will never be able to run them with tube amps. you know the rest of the story.
for classical music at low volume, I highly recommend high efficiency (95+ db) and SET amp. I can see myself listening to big Tannoy with SET glowing in the background and my face grin from ear to ear.
i own a pair of montana xp speakers they are rated at 92. i think that is pretty efficent and the sound very musical with any kind of music you want to play. they have awsome slamming bass. awsome midrange and treble
Are there benefits / disadvantages to either?))
Typically ext ream High efficient speakers with multiple drivers in the same pass band tend to achieve their
amplitude or efficiency at the expense of clarity by smearing which in my opinion sounds worse at low volume.
A well designed point source speaker with an easier load for tubes would be what I think you are looking for.
((I listen primarily to classical and prefer tube amps, does this lead me toward one speaker over another?))
I would take a look at the more well known mainstream successful high end designs that match your preferences.
((Lastly, I try to keep the music at a fairly low volume -- need to protect the hearing!))
Just remember a coherent speaker wont need the treble jacked up to balance fix the bump in the bass
If you like tube amps, a high efficiency speaker with a benign impedance curve is a no-brainer.