Using Denon 3803 as pre/pro a good idea?

I'm new to this website so excuse my ignorance in advance...
I've recently pulled my old separates (PS Audio 4.6/B&K ST140) and installed a Denon 3803 as the financial burden of purchasing full separates for HT was more than I could talk the wife into. I'm running B&W CDM 7NT's, a CNT, and 1NT's for surround with a Sony CDP700(?) via optical to the Denon. Movies are more than acceptable but music reproduction has suffered. I'm considering hooking up my ST140 to the front L/R and running in "Pure Direct". I'm mainly trying to improve the sweetness of the sound and dynamics that seem to be missing using the Denon. I should say that I've only had the system running for about 4 weeks so burn-in may not be complete. An audiophile friend opined that hooking up the B&K would be a bad idea, especially during movies, as it will present different amperage(?) loads (sorry, can't remember as he was waaay above my head) to the pre/pro while using the inline amps for center/surr. Any ideas or suggestions would be appreciated
I use a Denon AVR with B&K monoblocks M200 for the fronts pre out. No problem at all. The Denon just has that much more power for the ohter channels. For stereo I use a tube pre to the B&K switching it out from the Denon.
Are you actually unplugging your amps and speaker wires to your L/R to your AVR and switching in your pre-amp? What kind by the way? I'm hoping you have an easier solution.
I've got the 3802 and want to do the same thing.

Should I invest my whole budget on a quality amp with a lot of power for my Paradigm Studio 100 v2 fronts and use the pre-outs on the receiver.

Or would you guys suggest less of an amp and a new preamp to switch back and forth like sstark does?

I've got about $1500 to spend.

(or would you keep the AV like it is and add a better subwoofer)
I have done this with a 2801 and rotel/acurus amps and had better results, but then I switched to a sony 9000es pre/amp and digital sources sounded very good but music was still to bright. Installed line stage ps audio 6.0 pre and run the inputs into my 9000esn 5 channel amp's second input and there is no comparison. The analog pre is much better for music.
You don't need to do any unplugging. Just run a cable from the Pre-amp out of the Denon into any input on your analog 2 channel preamp. Hook your CD player directly into your 2 channel preamp so when listing to music, the Denon doesn't need to be powered on. Lot's of preamps have a Home Theater pass through but it's not necessary. Just set the volume level on your preamp to a predetermined level (unity gain) and calibrate your HT receiver.
God, things were so much easier before HT...
Terrific advice that I hadn't even considered, however one of the reasons I bought the Denon was to utilize its DACs as the Sony's are pretty frightening for music (hence the optical connection). I'm on the verge of purchasing a Denon DVD2900 (or similar product if any of you have a suggestion) and when I do I may go your route. Question: I'm not running separate DVD and CD players (out of space) so how do I get the signal back up to 3803 for HT or am I missing something here?