Audiobb, I am surprised the 300B sounded better than the Cambridge consider the fact that you were using B&W 602. Had you been using appropriate speakers, you may find your 300B capable of the same (or better) bass than the Cambridge.
+++ But these 300B monos could be improved with better output transformer +++
Possibly, but it's bad would most certainly be improved by speaker presenting a more benign load. As an analogy, think of using a 300B of B&W equivalent to diving a Ferrari on the beach ... not very smart.
Regards
Paul