If this helps any, I just got a HSU STF-2 this weekend and it is way more than enough for a bedroom. My bedroom is 10'x13' and it can easily shake all four of my walls while watching movies and I have the gain only at about 30% power. I had a cheap Onkyo sub and this STF-2 kicks it butt. I can hear so much more on the low frequencies and it is much tighter, which is great for music.
The STF-2 actually going to go in the bigger HT room downstairs that is more than double the size but I wanted to test it out in my bedroom system first. In your size room, the STF-2 is plenty loud (I doubt you'd use all it's power) and can easily handle a larger sized room if you move.
The 10" driver in that HSU is more than capable of rocking your room and keeping everyone and their dogs awake.
And yeah, the STF-2 will be more than fine with your Denon 591. That Denon has MultiEQ so it can EQ your subwoofer, too, which should help a lot. Plus it has Dynamic EQ and Dynamic Volume, so you can watch movies at low volumes and still get decent bass response. You might also want to get a riser for your subwoofer like the Auralex Subdude especially if you have wood floors, or carpet over wood floors so you don't want to annoy any neighbors, roommates, etc, when you are pumping your system.
