Impressive speakers, a sound purchase for sure. How many hours of playing time do your speakers have so far? And how many hours did the speakers in the store have? What Marantz did you listen to? In the store I work in, I've listened to the Karat 790DC connected to a Marantz PM7001KI and SA7001KI alot of times, and they never fail to impress me. A Harman Kardon AVR-745 which we have hooked up too can drive the Cantons pretty well, but doesn't really bring out the best in them.
From what I can remember, the speakers sounded quite a bit different when they were fresh out of the box compared to how they sound now. And if you like the sound of the Marantz, why not shop for a Marantz stereo combination? You certainly don't have a bad receiver, but a surround receiver is a compromise most of the time. If all you do is listen to music, instead of watching dvd's then I highly recommend in getting a stereo amplifier. Getting a dedicated cd-player as opposed to a dvd player will reap some huge benefits too.