High powered single end 300b recommendations

A 300B SET is going to give you 8-10w.  You can step up to a 300B XLS and get about 13w.  the 520B which is in the same family will get you to 16w.  A parallel SET of 520Bs will get you to about 28w.  

This is simply based on the specs of the tube.  What you can do is get a Push-Pull which will deliver between ~32-40w in power.  A good option would be KR VA830 which runs four 300Bs.  

If you research KR Audio, there is a site KRAudioUSA.com which is run and maintained by someone claiming to be the US Importer for KR that has an axe to grind with KR.  They do not have access to KR and never have.  

It is a good company that makes nice amps.  

You are married to high efficiency speakers if you plan on getting a 8 watt SET or even a PP 300b won’t be enough for a lot of speakers.   
 

I’ m driving a pair of Forte IV in a small to medium size room and it sounds great but push it too far and it falls apart. It will play moderately loud. I don’t listen loud so for me I’m using probably less than a watt.    These amps are not for people that like to play music loud, that is not what they are all about. 
 

There is some misunderstanding between amplifier power and speakers sensitivity. The combination of these two parameters plus listening room acoustic characteristics like the Critical Distance give the maximal listening volume. 
There is not direct correlation between speakers bass control, speed and dynamics with amplifier power. 
In many cases it depends more to amplifier schematics and power supply design. For example, in mid size room speakers with 94dB sensitivity can sound better with 845 SET vs 300B SET not because "8 watts is not enough" but because 845 SET has more energy storage in power supply, interstage driver transformer and strong driver tube (for example 300b)  vs weak (for example 6sn7) driver in 300B SET.

Allnic A6000 monoblocks are rated 60w using EML XLS tubes, 4 per side. They also have a toggle for negative feedback and no negative feedback, which helps with more difficult speakers. They sound fantastic if you can find a pair. 

I have Altec 604E speakers in a mid-sized room. The real sensitivity of these speakers is ~97db. I drive it with a 6 watt 300B DIY SET amplifier with EML mesh plate tubes. I did experiments with changing 300B B+ power capacity. I started with 100uF per channel.
Then I went to 350uF electrolytic capacitors per channel. Next time I added 130uF of polypropylene and oil capacitors.
Each time I got very significant improvement in bass control, dynamics, separation, soundstage. The difference was much more significant compared to changing James Audio output transformers to Hashimoto H30 which cost 3-4 times more!
In the end I added another 1600uf of electrolytic CDE capacitors per channel and got another level of clarity, focus and bass control. The same happens when I added capacitors to the driver and input stage B+.
The difference between the initial variant of my amplifier to my amplifier now is huge. It is like its cost has increased 10 times.
I can’t imagine my speakers are capable of such bass performance with any amplifier.
I don’t understand why producers save on power supply capacitors. Why instead of going to the 845, 211 tube models don’t build a good power supply for 300B or 2a3 amplifiers that had much wider bandwidth, less distortions and a lot of other advantages.