I believe the MV60 was the next model after the MV55. The MV60 was really good though, one review said it was better than the Premier 11 by comparison. The MV60 used EL34's for the great midrange, the MV60se used 6550's which gave up a bit of the sweetness for better bass slam.
Anyway, the MV60 is a stereo amp. I bought a 2nd one, then shipped both back to CJ who converted them to mono for me. Very reasonable I must say.
If they still offer that service you could do the same with your 55. You'd just have to find another 55. CJ was very good with making sure each amp had similar specs. They had to replace a couple miner things to make the amps match.
I kept the cages off my amps, my brother used to say I had a Frankstein stereo, referencing all the glowing tubes.
Joe, it would be interesting to hear CJ and ARC together. CJ sounded to me to have a nice dark background, where ARC has this sort of white background. Would they cancel, one be dominate.....