I have a similar speaker set-up (2 F3's, 4 SLX's, a C3 and a SUB12) on my Synergy HT. I have the F3's set as "large" and everything else set as "small." I drive them with a Yamaha HTR-5890 and use 14 gauge wire. The room is about 4000 cubic feet (20 x 20 x 10) with hardwood floors. Wire runs are about fifteen feet to the F3's and C-3, and around fifty feet to the SLX's. I did bi-wire the F3's using the A and B outputs, but that didn't affect the sound level. My normal day/evening listening level is around -30db to -25db. In night mode, I drop down to -50db to -45db. I have never gotten up to 0db, as the system tends to shake the entire house at about -5db.
I am not familiar with the Denon AVR-2807; does it have a speaker impedance setting? Also, how are you routing the receiver inputs? Using the best possible outputs from your sources (coaxial or optical digital)? Any attenuation points (line level settings) prior to the signal reaching the Denon's input ports? I know that on my system, I get various input levels depending on the source (cable box, DVD player, HTPC, tape decks, game consoles, etc.). Most are fixed level outputs, although a couple were adjustable. It took me a while to get everything balanced to eliminate drastic level changes when swithing from one source to another.
Keep in mind that the Denon might not be at fault if you are not feeding it properly ... GIGO (garbage in, garbage out!) Good luck in isolating the problem.