Hey,
First, my apologies since I mispoke.  It is not the sub cross over but rather the LFE crossover.  Now to backup a bit.
On the Denon, you can set the crossover filter for each of the 8 input channels, with of course the LFE filtering above and the other 7 filtering below (it does group pairs, for example the fronts, sides, and backs).
You can also set small/large individually (with the same pairing as the crossover).
As Caleb mentioned, there is also the LFE+Main or LFE only setting.  Given those controls, it functions as follows:
With LFE Only, then the LFE channel is sent to the sub (if present) along with any other channel marked small according to the crossover (below the crossover for LFE and the smalls).  When LFE+Main is set, then the LFE along with ALL other channels (present) are sent to the sub out according to the crossover (below the crossover for LFE and the smalls AND the large).  This means that the crossover is settable/used for Large when LFE+Main is set and ONLY the frequencies below the crossover are sent to the sub (the large gets the full range while the smalls are filtered).  Though a bit complex, it does make sense and gives lots of flexibility.

I did test this using the simplest configuration (direct stereo with sub) using a CD (no LFE).  With the fronts set to Large and the crossover at 40Hz (the lowest setting), there was very little output from the sub and definitely only very low fequencies.  At 120Hz, there was significant output from the sub, but definitely not anything close to the mids in frequency.

I have the 633Ts and like to run them small with the cross over at 60Hz, I set the crossover for the LFE to 120Hz to ensure I don't lose any information.
As Jack states, this is a matter of personal preference within my room environment, and experimentation is encouraged.

If nothing else, I hope this gives a bit better understanding of LFE, large/small, crossover filter settings, and LFE Only/LFE+Main (or the double bass on the Onkyo).  Of course, the exact way that these controls interact will vary across AVRs.  Open to questions on anything that was not clear.

Hey Ken,

Thanks so much for your post. This is really revealing information. From my seat, it would seem that the LFE moniker is not used as I implied it is in my previous post. Though it may be my opinion that the term LFE should be restricted to referencing the LFE channel in surround encoding, it may be just that: my opinion. Naughty AVR manufacturers! [:'(]

You and Jack have reitterated a point that trumps all others when it comes to setup: "Trust Your Ears" That said, if you don't know what the ramifications of your adjustments are (due to lack of familiarity with functionality and terms) then it can be difficult to achieve your desired sound.

Ironically, my intention was to clear some common confusion by citing textbook definitions. It would seem I've done the opposite!

 I'm still going to test the Onkyo, but I'm afraid I won't be able to post my experience until April 4 or 7. I'll be consumed with work until my vacation starts. Can't wait!