High powered single end 300b recommendations

This doesn't seem like a good fit. You can get 15-16W from a parallel 300B SET, but unless you listen at very modest volumes and have a fairly small room, an amp like this is probably not going to meet your expectations.

hope price is not a concern: 

https://www.triode.co.jp/english/TRX-M845/index.html 

or this one, do not let the ratings fool you, this one will do the job.  

A true SE amp 300B is only 8 wpc , if you go into pure class A around 18wpc 

25 wpc in pure class A solid state like Pass Labs you can operate your Harbeths on .tubes maybe a Ayon Scorpio around 40 wpc in Pentode which is cleaner and better then ultralinear  tubes.