I have an Ultimate 70, it has no problem driving essentially any speaker to acceptable levels with reasonable dynamics. I have used speakers as low as 87 Db/watt (Ohms Walsh 100) with it, it was fine for my relatively small room, 90 Db/watt (B&W Nautilus 803S) is pretty good and by baseline system.
On the other hand, the noise floor is extremely low, so high-efficiency speakers have no real problems, either. When I finished my U70 mod, I hooked it up and flipped the switch, and *nothing*. I figured it either blew the fuse instantly, or was about to start emitting smoke, but no, I could hear the tubes clinking as the mica supports scraped up the side of the glass. I hit "play" on the portable CD player I was using as a test source, and bam, working perfectly. Point being, it's nearly silent, as good as I have ever heard from any amp, much less a tube amp.
I am no particular fan of horn or high-effiiciency speakers, but I would recommend it very highly for that, it's as good as I have heard in regard to parasitic noise.
Brett