Best 300b single-ended Tube amp. for the $

I am afraid I am not a horn fan, but there are a number of companies making SET friendly speakers, which are not Horns(I have been on a similar hunt). If you can find 15watt + amps, you will make the search easier. I agree with darkmoebius, there are other SET valves you could consider. I personally prefer 845's, always with the proviso that amp design and component quality makes more difference than tube choice. As you know, output transformer size and quality is key.
Speakers to consider that are SET friendly:
Silverline
Daedalus
Reimer
Zu
Coincident
Zingali
The one I went for is not an obvious choice at 89db sensitivity, but a flat 6ohm impedence, makes it SET friendly. That is the Acoustic Zen Adagio, at the price, it is a steal. It needs 50+ SS watts, but 20 SET watts drive them just fine.

Saffy
The Zu Druid or one of the cheaper horns, like the Cain and Cain Abby, might suit your needs. The latter sound and look great, maybe not ideal for Mahlers 9th, but wonderful for Jazz, solo singers etc