I spoke with with Steve at Outlaw.

I was told that the bottom plates are stamped, and because of the space for the terroidal, they actually "want" to curve a little. He suggested (if I felt comfortable going at them with a screwdriver) loosening the three screws along the back of the amp, adjusting the plate and then retightening the screws--but not too tight. This didn't make a much of a difference. Steve offered to replace the worst of the bunch. I haven't decided if it's worth the effort.

I still wonder if this is something that happens with age/use of the M200, as I think I would have noticed it when I set the amps up.