Which speakers are higher efficiency?

Gotta say I agree with Johnk and Gregm on this one. You need at least 95dB to make those 300Bs work really well, and I would almost say you need 98 (though it would depend on the impedance curve (and at that level, phase angle may come into play :^) but I'll defer to others on that)). Anything below or with a large impedance range with the low impedances in the bass and you will be risking serious disappointment (especially in the bass). I think the Coincident Total Eclipse are good high efficiency speakers in the 'box speaker' format, but my heart belongs to horns, which can come in all kinds of flavors. I'd think for $4-6k you could do yourself very well, either by buying something used off the 'gon or buying speakers custom-made for you direct from the manufacturer (disclosure: I've never purchased any speakers from any manufacturer who mentions himself on this thread (or is likely to going forward for that matter) but I've seen some of his creations in photos. Other builders are those who deal with BLH (back-loaded horn) designs. You can check Frugal Horn for some DIY designs, though there are some builders who will build a set for you, sometimes ones which are tweaked/improved beyond the original design. You might also dig around the archives here and over at AudioAsylum.com (the High Efficiency Speaker Asylum). There will be lots of ideas in both places.

Most 300b amps have 7 watts output or so. Is your amp an SET (and therefore class A2) or is it push-pull?

If an SET, it is essential that the speaker have higher efficiency. I am running a 30-watt amp on speakers that are 98 db, in an average size room (17x23') and that seems to be about the minimum amount of power to get by. With SETs in particular (my amp is an OTL) you don't want to be driving the amp hard if you really want to hear what it can do (which will be good low level detail/transparency). That will require a speaker that is 98 db at the minimum, 103 would be nearly unlimited headroom.

That is why you see so many SET users using horns.

Plan B- if you really are considering a choice between these two speakers (both are excellent- FWIW I prefer the Ref3a in this case), then you will want to get a more powerful amp- one that has at least 100 watts. The 10 db difference in efficiency of speakers (91 to 101 db) translates into a 10X power difference.

Agreed with JohnK, neither two speakers you are a good match for a SE tube amp. Sensitivity is only one factor you must look at; more importantly is a benign impedance curve.

You should consider one of Johns speakers (KCS), Cain and Cain or Carder sound (and many others)

Regards
Paul

Agree with the posters saying you need 98dB efficiency at a minimum for that type of amp, especially if it is single-ended, in which case you will want even more efficiency if you are in a large room. Horns match very well, as others have said. You certainly don't need to spend 4-6K, either, if you don't want to - I got a very nice pair of Klipsch Cornwalls on this site for only $600. You may need to spend a little more than that, but certainly not 4-6K. Anything in the Klipsch Heritage series would be good, or something similar.

I run a pair of 300b 8 watts SET with Coincident Triumphs Extremes and two passive subs with wonderful results filling a room 20x23x9
94db@1meter but most important a flat impedance curve of 8ohms