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.