300B Tube Amps with 15/16 Watts Per Channel?

Check out Audio Note Kits website. I know they make a parallel 300b.

I built a parallel 300B SET based on an ANK kit with a lot of modifications. I don’t have much experience with a single tube SET, but my amps sound very nice. Check out my virtual system. 

What you want is a Parallel Single Ended 300B amplifier...Look at:
 BorderPatrol S20EXD...
Fezz Audio Lybra...
World Audio Design...
Audio Space Reference 3.1 & Audio Space AS-6M...
Manley Labs Neo-Classic SE/PP 300B.........

You could also do a pushpull 300B amp that will get you closer to 25 watts.

Spatial Audio lab produces dual 300b  monoblocks designed by Don Sachs, ‘Blackbirds’  24wpc. 
 

If you don't mind assembling a kit, Bottlehead makes a 300B model called the Kaiju that can be configured to work as a 16W monoblock amp. I have the Kaiju configured in stereo mode and am very pleased with its sound quality. I can only imagine that the Kaiju in monoblock form would sound just as good or better, but with more power. Bottlehead kits are very complete with superb instructions, which makes for a relatively easy build.

Absolutely one of the best is Aric Audio. Aric makes a 300b parallel SET amp, the Super 300b PSET.