I have used the XR55 with three speakers - low efficiency 2 way with RS225 and TB871, fullrange with CSS WR125, and fullrange with TB871. In all 3 applications the panny pushed them to satisfactory volumes, though I never REALLY pushed the big 2 way, as the setup was in a townhouse.
I would think anything over 6ohms, 84db should be fine. But then again tastes and rooms vary. Certainly anything a cheaper 'normal non-digital' receiver from sony, yamaha, denon, or pioneer can drive, the panny should. Most of these receivers don't come close to their rated power, and very few of them can drive a full complement of 4ohm speakers. The panny definately puts out more clean power than my pioneer 811s, or my friends' low-range yamaha and onkyo receivers into the speakers mentioned above.
Finally, not all mtms are 4ohm, some mtms use 16ohm drivers and end up with a 8ohm impedance. For example the some of the RAW HT series kits and some of the exodus audio kits.
