I can't comment on the power gain or the sound in the Cary directly (so my reply may not be terribly helpful!), but I have done a lot of 300B comparisons now in other setups and Idid want to say that the 300BXLS tubes are truly remarkable! They are amazing top-to-bottom performers and tremendously musical without being euphonic. Any additional power you may gain (assuming you can adjust bias as they describe), will be an added bonus - I had no power gain in my VAC amps, but just enjoyed the performance.
Good luck - I hope you can make them work!
Jacob
Good luck - I hope you can make them work!
Jacob