Well, your speakers are not terribly hard to drive, being 8 ohm, 89dB sensitivity. They are only recommended to handle 80 watts. The Denon receiver you are using has a very capable amp section that far exceeds any requirements of the speakers. I fear that adding an amp with these speakers will produce no tangible benefit whatsoever.
Moving over to the pre/pro side of things, unless you need HDMI switching and the new hi-def audio formats, you also stand to gain little from replacing this Denon, which has a very competant audio processing section as well.
If your goal is simply go with separates just for kicks and giggles, you will be hard pressed to find any component for $300 that will do a better job than the 3805, even using Audiogon. Far be it from me to tell you how your system should be arranged, but were I in your shoes, I would apply the whole budget to a better set of L/R mains for the greastest improvement in sound quality.
If you go listening to speakers and fall in love with a difficult set to drive (low impedance, low sensitivity), then let the requirements of the speakers dictate the type of amplification you need. If you find that you require more features in a pre/pro than the Denon offers, then let that be the deciding factor in that choice.