Speakers under $700 for 8W Per Channel Tube SET

With that room volume you will probably want 95 dB+ but much depends on your maximum listening "volume", minimum impedance, impedance variation (curve), type/presence of crossover and your listening position in terms of distance from the speaker. I have a second system moderate size bedroom (maybe 10 or 11 x 14) with the speakers on the long wall. They are front-loaded, single driver zHorns at about 93 dB efficiency and I listen to singer songwriter/folk/acoustic blues, etc, at low-moderate volumes. I tried a Berning microZOTL (which is 1-2 wpc) since I was mostly using headphones and found that it works in that situation. I'd like to have a bit more gas but since the music does not have much deep bass and the volumes are nowhere near "realistic", its just fine. Your MMV.

Coincident or Klipsch

Someone is selling a pair of 3a deCapos with stands at $1K right now. If you can stretch to $1K, I believe that they'd be a fine choice, particularly given your taste in music. I used these speakers successfully with Cary 300B mono amps in a room just about the size of yours. The specs on these speakers don't indicate what a fine match they are for SET amps. There's a single cap hanging off the tweeter instead of a full blown x-over and, in practice, that appears to make all the difference in the world.

Marty

You might also consider the single-driver Omegas - perhaps the Super 3 (or 5) XRS in your price range.

KEF Rf 104.2 or JBL 4312