High vs. Low Efficiency Speakers

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

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.

To quote Ralph Karsten of Atma-Sphere: "...if you want a tube amp, even if you don't want an OTL, you should avoid four ohm speakers. " cf. "Debunking Common Power Amplifier Myths"