Sounds like you may have an amp channel going bad.
I would check out all the speaker wiring to the Denon and the speakers internal wiring as well...you never know...
You can most definitely bypass the internal amps and use the 3802 as a preamp -- I own one and am doing just that right now for the exact reason you would be!
Semi-funny story -- I purchased my 3802 (refurb off eBay), and within the first 5 minutes of unboxing it I made a massive bonehead move that still haunts me to this day.
When hooking up my Parasound HCA-1205A 5-channel amp, I somehow (still am not sure what I was smoking that day) plugged the interconnects from the Parasound into the multi-channel inputs on the Denon instead of the preamp outputs. Oops. Puff of smoke. This started the fun.
The nearest authorized Denon repair center is 350 miles away. I happen to travel there for business monthly, so I figured I'd just drop it off to avoid shipping. What I figured would be a simple 1-month repair turned in to a 4-month nightmare! After numerous calls to see if it was done yet, I finally got it back after the four month wait, drove it the 350 miles back home, hooked it up, quadruple-checked all connections, powered it up...and the right side surround channel is distorting.

Very angry now...
So, I have to wait another month to drive it back. They take possession of the 3802...for another 3 months!!!
The frustration level was off the chart during the wait. During the 7-month period I suffered with a 9-year-old Pro-Logic replacement receiver. But I adopted the "s$%& happens" approach, and figured I finally get to enjoy it now, so what the heck. I did not even check to verify operation the internal amp's operation this time, figuring there was no way possible it couldn't work. I just hooked up my Parasound and enjoyed it for 18 months or so...
...which brings me to last week. I disconnected the Parasound as I was planning on selling it, and hooked up the Denon's internal amps. During calibration test tones, I was absolutely sickened when I heard no output coming from the right side surround speaker. Then the left side surround tone came on...and there was output from the left
and right side channels! I thought my head was going to explode. Ripped the cover off, traced the wires, and found the "repair tech" had snipped the connection to the right side surround's amplifier, and
jumped it's speaker connection to the left side speaker's binding post! Beyond unbelievable.
So two weeks from now, I get to drive back and present my findings to the manager of the shop...and try to prevent myself from ramming the 3802 down the tech's throat if he still works there!
Sorry, that ended up a bit long but it feels good to vent. I have heard many horror stories about Denon's authorized service centers...so I highly recommend just getting an outboard amp and using the Denon as a pre/pro!
Aaron