Thanks to all who responded so far. I emailed KEF with this concern, and this was their reply:
The Metas arrive tomorrow. My plan is to stick with the PM7000 and see how it goes. There probably won't be much of a need to crank up the volume so much as to damage the speakers or amp as I'll be using these in my fairly small study.
Of course the main caveat here is that KEF's main interest is in selling me speakers.
"The Marantz you are looking into would be a good pairing. We list the LS50 META as an 8 ohm speaker because the system has an 8 ohm impedance. This will not affect the Marantz even when the impedance may drop down under load.
The high end of the power range we recommend is 100WPC into 8 ohms. Since the Marantz will cover 95% of this it will be a good match."
The Metas arrive tomorrow. My plan is to stick with the PM7000 and see how it goes. There probably won't be much of a need to crank up the volume so much as to damage the speakers or amp as I'll be using these in my fairly small study.
Of course the main caveat here is that KEF's main interest is in selling me speakers.