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

Saffy,

Decide on high efficiency speakers first because they will be the biggest determining factor in a low-power system depending on space, musical preferences, and replay volume. A better understanding of your preferences/needs would really help us with suggestions.

If you are into Mahler at realistic volumes with the least amount of distortion and greatest dynamics, a 100+dB multi-way compression horn system would probably be best.

Intimate acoustic, jazz trio, quartet, vocals etc...that's where full range single-driver systems excel.

In between, multi-way compression/dynamic hymbris and SET-friendly multi-way dynamic speakers like Silverline. etc.

The musical presentation is so incredibly different between those 4 major categories, and even within them, that you really need to arrange to hear other people's high-eff setups before making the jump.

As for amps, depending on the speakers you choose, you may not have to limited to 300B amps. There several other fantastic output tubes that equal, or even exceed, 300B's in many ways.

You've already heard and marveled at an Art Audio amp. The owner, Joe Fratus, considers their PX-25(which I own) to be their finest. It has 6wpc output, but out drives many 300B or other higher powered SET's. These sell here for $2,500-3,200 used regularly and hold their value at that price. Also consider their 845 based Carissa.

Welborne Labs 300B DRD or Terraplane monoblocks are amazing. DIY HiFi Supply's Lady Day+ monoblocks are great and have a large following.

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.

The hard part is finding new high efficiency speakers in his $2,000-3,000 range that will properly fill a large room.

Used though, opens a pretty large selection:
Klipschorn/La Scala types
Silverline Sonata/Sonatina
Von Schweikerts
Living Voice Auditoriums
Classic Altec Voice of the Theatre style Valencia's

Hi
Really liked what I saw on the 6-watt PX-25 fron Valve Art..sold on SET now

My monitors are 89db @ 7ohms,though if I added a powered sub wouldn't that compensate a little power distribution-wise?

No one has metioned the new Celius esw from Triangle (92db)

Horns intrigue me - but know nothing about

I'm more into Jazz than anything

"My monitors are 89db @ 7ohms,though if I added a powered sub wouldn't that compensate a little power distribution-wise?"

Not unless you included some type of "high-pass" filter before the amp to block low frequencies from reaching it. Even then, it may not work depending on the impedance curve of your speakers.

Bottom line, your speakers probably were not designed to be used with low-powered SET amps, so don't waste your energy trying doing so. That would be an attempt to use the wrong tool for a specific job.

The goal is not have a system that simply works, but to have one that works to the best of it's abilities. Your speakers will not provide that with lower-powered SETs.

On the other hand, Art Audio's 16wpc Carissa (845 tube), or even better, 20/32wpc Jota (depending on output tube) probably will drive your speakers fairly well. But, they do not have the same "magic" of the lower powered offerings. Although, they do have their own benefits in the area of lushness and slam, respectively. Either of these amps could potentially make a fantastic match with your current speakers.

Please heed the advice people are trying giving you here, low-power SET systems requirement far more planning and understanding than normal high power setups. There are no shortcuts, there are no free lunches.