Well, there you have it, the EL34, 6550, and KT88 are the best with the Music Reference.
Actually this is true. Depending on the rest of the system, the room, your preference in music, and so on, each tube's characteristic will slightly tailor the sound.
I keep my pair of Quicksilver Silver 60s for their wonderful midrange presence and classic tube sound courtesy of EL34s. My RM9 on the other hand just kicks butt with the RAM KT88s. More of everything and at the same time more neutral and NOT tubey sounding.
I just bought a barely used original Rm-10 for my office. With only 200 hours on the amp and the original RAM tubes I've got to tell you it does sound like a mini RM9.