Depending on room size and music choice, you will want high eff. speakers with a 300b amp. As Atmasphere and others point out (and I have heard w my own ears), once you start to push an SET amp, distortion increases dramatically. You will want at least mid-high 90's dB/w/m efficiency. In my 14' x 23' x 8' room, I could easily hear 8-9 wpc 300b SET starting to distort at moderately high volumes, even with relatively undemanding material. If you listen loud and/or full orchestral music, you are probably want to look at high 90 dB. Also 8 ohm or higher, flat impedance curves. Names that come to mind are Coincident, Horning, maybe some of the Ref 3As if you room is small enough, Deadelus, Rethm, Zu. Probably some of Duke's Audiokinesis line. If you actively bi-amp or use a sub with the signal high-passed to the mains, you can probably go down a bit in the efficiency department. As johnk points out, the laws of physics are a bitch!
Speaker Suggestions for 300b SET Amp
I built an Elekit 8600 300b set amp recently. It's amazing. I maxed out upgrades (all Takman resistors, Mundorf top end caps, solder/wiring, etc.). It's a wonderful amp. It cost me $2k to build it and it's glorious.
My issue hasn't been the amp, but finding speakers that I want to use with it.
I started with Omega XRS 8 Juniors (~$2k). I broke them in for 300 hours and, while they did get better, they largely sounded like the top and bottom ends were just truncated off at both ends the sound spectrum. They sounded more akin to a transistor radio speaker. I feel terrible to say that and I hope others really love them and hear them differently, as the company is great and the owner is wonderful. They just weren't for me.
So, I just paired the 300b up with some inefficient speakers (both KEF LS50s and Wharfedale 80th anniversary Dentons). Both speakers were part of other systems I have. Both sets, and especially the cheaper Wharfedales, just sing with the 300b. They do not play particularly loud given their efficiency ratings, yet they sound wonderful for very close nearfield listening.
But what I'd like to do is go a bit more into the full range speaker category without buying a massive product. Or, I consider a smaller bookshelf/monitor if it were more efficient.
What speakers do folks like with their 300bs?