As far as recievers go, that Denon 3803 is fantastic for HT, great choice!
something you might want to try is a KEF speaker setup. I had Kef Q1's runing with my 3803 and they worked together extremly well, i was very happy with the results. The room was similar sized too, about 13ft by 26 feet. They would have no problem in your room. They have very good imaging for monitors as well, and can reach down to 45hz suprisingly well.
The KEF Q1's go for 225 each, if you get 6 of those and the Kef center channel, with a good subwoofer you will have a fantastic 7.1 HT setup.
Velodyne subwoofers are pretty good, I think that Definitive Technology subwoofers go neck to neck in performance and cost quite a bit less. I was using the Definitive Technology Prosub 200 in my rig, the 250W amplifier gave great bone rattling bass throughout the room! :) Those babys go for about 549 new. So they arent TOO expencive.