The New Western Electric WE91E 300B Integrated amp ?

There is no way you can get 20W from a pair of standard 300Bs. I owned a superb Mastersound 32B integrated that I used just as an amp that claimed 22 watts and it only really put out an acceptable 14. The 32B is a very hot 300B variant. At 20W I would guess well over 10% distortion for this amp.  

Actually the way SET amps distort is precisely why you should be somewhat interested in distortion numbers. After hours of comparing OTLs, push pull, single ended and parallel single ended you start to understand exactly why SETs seem to sound so good when they are in fact clipping. Much of the warmth and bass you hear is soft clipping and it is not offensive but it is there. Having said this I still prefer aspects of the single ended sound to many P.P. or ultra-linear designs.