I know that Ron Welborne announced several years ago that his Moondog Audio was going to sell active speakers powered with 300Bs. It looks like Moondog never got off the ground (although Welborne Labs remains alive and kicking), so I suspect that only prototypes were built, but I'm not sure.The link is to an old Welborne Labs Lab Report. Go about one-third of the way down the page for a picture of Ron's "300B Self-Powered Speakers".