I have the Studio 60's, Studio 20's and Studio center channel and currently use a Rotel 985 5-channel amp (100 Watts/Channel). It has plenty of power, and much cleaner than when I was using the amp in my Yamaha reciever. I would suggest going with the RMB-1075 since it's the newer version and has slightly more power. My room dimensions are similar to yours as well.
It is also possible to bi-amp the 5-channel amp to drive your 2 front speakers, use the other channel to drive your center channel, and use your Denon to drive the rear speakers. This worked fairly well in my set-up before getting rid of the Yamaha and upgrading to a B&K ref 30. I still really like the Rotel and have no plans to upgrade the amp.
It is also possible to bi-amp the 5-channel amp to drive your 2 front speakers, use the other channel to drive your center channel, and use your Denon to drive the rear speakers. This worked fairly well in my set-up before getting rid of the Yamaha and upgrading to a B&K ref 30. I still really like the Rotel and have no plans to upgrade the amp.