It will depend on your room and listening tastes as Greg states. It will also depend on the amp and speakers themselves as the spec's/numbers are just a rough guideline. I have an Audion Silver Night 300B SET that does not seem to care if the speakers are of the low impedance type (part of the design I would imagine). I run a pair of Reynaud Twins (90db at 4 ohm, that could have been rated, I was told, at 92db @ 8 ohm, again the spec's) in an 18 x 24 room without any difficulty. Other speakers rated at 90db @ 8 and 4 ohm have not faired nearly as well. The RMS rating of the various 300B amps can also be somewhat misleading as many of them are rated at enormous levels of distortion, 2nd harmonic, (a level/rate that you would not be happy with, if you pushed them this far and the volume and output will most likely be kept much lower). Generally speaking though you are looking for speakers with high efficiency 92+ db and a smooth impedance curve that does not dip too low when under a full (real world) musical load, if you listen to R&R, full scale classical or other demanding music at above moderate levels. Anyway, try before you buy, if at all possible as system synergy has as much (or more) to do with the overall sound than the spec's do. My next speaker will most likely be the Hammer Dynamics kit that is rated at 96 db (I think). The kit is under $600 (though you have to build the cabinet or have them built yourself) and it has a strong following (these are floor standers). Other monitors that work well (in a reasonably sized room) are the Triumph Signature by Coincident Technology and the Soliloquy 5.0. They all sound different and I of course prefer the sound of the Reynaud which is a bit warmer. The Triumphs offer up the best bass of the bunch and the 5.0's were the most transparent. I could really be happy with any of them in my setup.
