There's a way around it. This applies to the 3801/2/3 and A1SR (5803). Copied and pasted from this link.
http://www.audioholics.com/productreviews/avhardware/Denon-AVR5803p4.html
"After some thought, Denon tech support gave me the info on how to accomplish the bi-amping task. Here is how to do it. Note this will only work if you don't plan on using the amps for surround back channels or multi zone 2 applications.
Bi-Amp Configuration of Main Channels
Step #1: Connect a pair of analog RCA cables from the preamp outs of the Front channels to an unused input of the AVR-5803 (In my case, I chose VDP).
Step #2: Select the chosen input as the source for Multi Zone 2.
Step #3: In the Multi Zone Control menu, select power amp assignment for Multi Zone 2.
Step #4: Using the internal test tones of the AVR-5803, position the SPL meter (slow response, C-weighted) at the listening position and increase the master volume control until each speaker reads about 75dB.
Step #5: Disconnect the speaker level connections from the Front channels at the AVR-5803 and reconnect them to the appropriate Multi Zone 2 speaker level connections.
Step #6: Repeat Step#4 while varying the Multi Zone 2 volume level. I found nearly unity gain correlation between the Front channel and Multi Zone 2 amplifiers to be at the Multi Zone 2 volume level setting of +2dB.
Step #7: Remove the jumpers on the back of the main speakers (you would be amazed at how many people forget to do this, Yikes!)
Step #8: Connect a set of wires from the Front channels of the receiver to the highs section of each of your front speakers and a set of wires from the Multi Zone 2 channels of the receiver to the lows of your front speakers."