My Denon 2910 DVD player allows me to set "source direct" which will bypass it's 80Hz crossover sending a straight source to the 990. Wouldn't this be the way to go since the 990's bass management is always on with speakers set to small?
I'm not sure when the 2910 uses that source direct mode. It would
not be doing any bass management for digital outputs no matter what, since it is simply passing the audio bitstream without any manipulation. For DVD-Audio and SACD and any other analog output, however, it is decoding and converting to analog, so it does have the opportunity for bass management. If that is the avenue where source direct is available, I agree that it's probably a good idea to use source direct and let the 990 handle it.
Also, what is the difference between setting my crossover to 80Hz and setting the speakers to small. I would think setting 80Hz would in essence make my speakers small as they would not receive lower freqs. In other words, what would be the difference between having my crossover set at 80Hz and having speakers set to small or large?
With the 990 (and the 950, 1070, and 970), setting a speaker to large or small is the first step in configuring bass management - you also select the crossover frequency that you want to use. Both steps are important. "Small" means "use the crossover point I select" while "large" means "I don't need no steenking crossover" and the crossover point is ignored. If the crossover is at 80Hz and the speakers are small, then the 990 crosses over at 80hz for those speakers. If the crossover point is 80Hz and the speakers are large, then the 990 does not cross anything over.