Adcom vs. McCormack? HA, wow that is one strech.. There is not a SINGLE Adcom capable of even coming close to a McCormack amp of any line they have put out sorry.
But anyway Plinius is a nice amp, but its a bit dry and pricey, I had one vs. a CODA class A amp, the Coda was easily on top.
Plinius vs. McCormack could really depend way more heavily on the absolute system synergy and speaker its driving.
Disclaimer I own none of this equipment anymore or these brand names. But for the record the Adcom was the bottom of the heap :-)