Hi RSA,
I owned a pair of Maggie IIIa's for years with a BK 202ST, with similar or somewhat higher power rating as your Marantz. I was never that happy with the set up thought the amps could drive the speakers to very loud levels. At some point I borrowed an Adcom 5802 high current jobber (years ago, so I may have the model wrong), and was amazed at the improvement in dynamics and detail...at the levels I listened, I think I was missing 10-20 percent of the music with the lower powered amp. I am not recommending Adcom since I have never owned any of their gear, but am definetly recommending auditioning different amps with the Maggies or at the very least auditioning the speakers with your Marantz before purchase if you are tied to that amp. The MGIIIa's were much better than I thought when paired with a complementary amp.
C.