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.
