My favorite tubes when I had my balanced V4’s were Ei KT90’s and GE 6L6GC’s. The 90’s were more powerful than the 6L6’s but the vocals with the 6L6’s were outstanding. My audio dealer likes KT 77’s in there now and he’s not a fan of the 150’s. The V4’s are the only amplifiers that I regret selling. Good Luck!