Cain-Cain speakers (any model), Decware HDT (I have a pair and love them), Avant Garde horns, Horn Shoppe horns, almost anything with Lowther drivers. Really there are many great, simple speaker systems that work well with low power tube amps. If you like making your own amps with kits I recommend the Transcendent Audio kits or Dr. Bottlehead kits.
Have fun.