If you need affordable, high-performance speakers for a SET and don't have the money for full-range horns, try the Super12's. One of the biggest problems with SET amps is getting sufficient bass at low power and still retaining the magic mids/highs. These puppies go to 40Hz and are 97db efficient! They'll go loud and deep on just about any music, especially if you are using a 8 watt 300B SET amp, plus you'll have all the SET magic you could ever want.
The only slight drawback is that they come as a simple kit for the drivers and crossover, you have to build the boxes(or have someone do it). Anyway, the kit is only $600 and the speaker boxes can be made for about $60-100 with simple MDF boards. It's dimensions are 14.5" x 14.5" x 48".
Check out the reviews on the Hammer Dynamics website and also the Super12 modifications page. There are quite a few people who have taken the performance to near reference level. You can email or post them from the Single Driver Website that hosts the Tweaks page. Search the archives on the Single Driver forum for posts. Also check Audioasylum.com
BTW, they'll work great with any amp, SET, push-pull, Ultr-linear, solid state.
http://www.hammerdynamics.com/
Super 12 Tweaks:
http://melhuish.org/audio/super12.htm