I haven't explored the source direct function yet. However, using the multichannel analog output, you can either set up the bass management in the 5900 and turn the Outlaw's switch off, or, as you suggested, set the 5900 speaker settings to large and sub on and turn the Outlaw's switch on. You won't be getting multiple crossovers in either case.

There is a lot of "tweakability" with this machine and I've only had it for a couple of weeks. I've been using the Denon's analog output for movies as well. Give it a try unless you're running 7.1