Robert,
Welcome to the forum.
The Denon and Marantz sound very similar, with the Marantz being a step up. Now, the 7500 IMHO compares more to the Denon 2807. The Marantz SR8500 would be more equivalent to the two Denon models you are considering.
If you are looking for a big change in sound, I don't know if you are going to get that with either Denon or Marantz. There will be a bit more headroom, but both units are pretty much flat sounding out of the box. Have you demo'd Rotel? That would definitely wake up your system. It's a bit more sibilant, but not in a harsh way. Most fancy features on an AVR go unused. I've got the Denon 3805 (it's a powerhouse, with tons of features), but what it has to boil down to is sound.
If I had to choose between those three, I'd go the Denon 4306. You've got some awesome towers, and they deserve a great AVR. I'd also look at what Rotel had to offer in a similar price range.