First off, do not go and blindly start adding room treatments, such as absorbion panals and bass traps. Before you do that, you need to invest in a measurment device to easure your room acoustics and see where the problem area's are at, then based on those measurements, you can start tweaking the room with panals and traps. As far as your current pre/pro and amp, you have very good combo of Rotel gear that should work just as good as anything else that you can buy for 8k. Amps pretty much all sound the same, and, yours have more than enough wattage and current. There is a very small amount of difference in sound quality between pre/pro's, but, not enough to make a big enough difference to warrent an $8,000 dollar expendature. I personally think you would be better served by either, a) getting a stand along room correction devise to custom EQ your room and speakers together, or b) sell the Rotel pre/pro and go with a pre/pro with room correction built in. I also think that you would have a freaking HUGE jump in performance by taking that money and adding a B&W htm1d, as that is one of the greatest cental channel speakers ever made. Some might say that you need to go with a htm4d to match your front 804d's, but the htm1d will blow the htm4d completely out of the water! Plus it will match just fine with your 804d's as they are all voiced to work fine together.
If this were my system and my money to spend, what I would do is, first buy the htm1d (if you can find one, they are discontinued and were replaced with the htm2d, which isn't near as awesome as the htm1d) as that is going to turn your good setup into a monster eating bad a** setup, then look for some sort of room correction devise or possibly use a Behringer or QSC signal processor to use as filters to tweak your mains and center as well as your sub(s). If you did all of the stuff I described above, then you would have a system worthy of any audiophile on the planet!