Should Amps be plugged into a power conditioner?

Depends upon the amp and the conditioner.  Conditioners that allow many amplifiers to function without limiting the headroom are very expensive.

The amp's class of operation could be a factor, e.g., class A, AB, or D.  Any knowledgeable opinions?

For me better safe than sorry. Panamax had been my best equipment "shepherd" and protector since deep early 90's and never failed even with STRONG surges. Never cared to upgrade that one.

When I was at a local dealer hosting McIntosh, one of their amplifier engineer/designer was there, and said amps should not be plugged into a power conditioner. The Power Supply, properly designed, does not need it and it lowers the draw rate of the amp. The amp should be plugged into the wall. Pre-amps and sources SHOULD be plugged into power conditioner(s) since they pull power more consistently.

If you’re using a power conditioner for your amp, try plugging directly into the wall and see if you notice a difference in SQ. I did.