I know magazine reviews are not exactly impartial but I don't think I've ever read a review of a Meridian amp that's raved about them.
Sue Kraft of Absolute Sound wrote a glowing article about the 588, 502 and 559 combination. From memory, she preferred the 559 of all the units in isolation. She was going to buy the review samples but held off and bought the G series instead. I know it's only one review, but it's a good one for M amplification.
I've never read a bad comment about 557 or 559 from users and dealers that I've spoken to who know them rate them very highly indeed. I have the 559 and couldn't be more pleased by it - it runs cool and has massive reserves. Above all, it does that "disappearing act" that all great components seem to do.
Hope this helps.
Regards,
Steve