Had a look at the MastersounD 300b specs, the 15w output in class (A) is very optimistic for a class (A) amp, it would be possible in class (A2), most good 300b amps are spec'ed at between 6w to 8w max, depending on your room size and how loud you listen you could get away with a 2a3 set amp putting out 3w, so a 300b amp set amp giving 6w output would be the other option. Have a look at <Bottlehead.com> these are kits to give you an idea, no affiliation with this Co.