The 650m will have a fuller, heavier bottom end and a little larger soundstage. I would know, I've sold them new. mc and mcx versions are more articulate, clearer sounding. I sold them new when they were current and have sold many over the years.
350m became 350mc then 450mcx
250m became 250mc then 350mcx
650m became 650mc then 750mcx
The transformers and chassis size are all the size within each model upgrade. The mcx version modified a driver board to push another 100 watts out of the amp. It worked. Never confuse a 350mc with a 350mcx the 350mc is a much better (and larger amp)
350m became 350mc then 450mcx
250m became 250mc then 350mcx
650m became 650mc then 750mcx
The transformers and chassis size are all the size within each model upgrade. The mcx version modified a driver board to push another 100 watts out of the amp. It worked. Never confuse a 350mc with a 350mcx the 350mc is a much better (and larger amp)