I wouldn't say that. I own a block letter 5150 and a 6505+ (5150II) right now. The 5150 has a greater low mid punch over the 5150II/6505+. The II is tighter and more articulate and high mid voiced. I wouldn't say it's fizzier, although the high mid voicing may make it sound that way. I think the 5150II/6505+ has an easier time copping the regular 6505/5150 tone, whereas the 5150/6505 can't reach the level of tightness and articulation of the II/+.
the +/II definitely has less gain, but it's not drastic. Both have more than enough, anyway.