Usually it's recommended to set the crossover point 1 octave above the extension of the speakers, OR 20-30Hz above that point. Crossovers blend the signals on "slopes", so you'll still get signal to the speakers well below the crossover point. THX spec is 80Hz, and many, if not all, sound mixing engineers will be using that in their own studios to mix the soundtracks for the DVDs.
You can do it arbitrarily. Or, you can take some measurements with test tones and an SPL meter and see what gives the best performance (flattest response). If you get the sub and speakers integrated well, you should be able to switch between the mains only (Large) and the sub + Mains (small) and not notice a difference except depth of the extension the sub adds. If you have a disconnect, where the sound is disjointed, it could be phase issues.
Then you have to see how it sounds to your ears. Doing tests, I found that 100Hz for my center channel (I have an HK as well) had the flattest response through the crossover region. But on certain male voices, some of the bottom end of the voices were coming from the sub. I lowered it to 80Hz and that is gone now. (BTW, I have my Ascend speakers crossed at 80Hz on my HK AVR330).
I assume you've already checked which phase setting on the sub works best too (it's not always 0degrees - 2 times with 2 subs in 2 rooms, with the sub in the front of the room, 0 degrees has given me lots of phase cancellation and 180 degrees is pretty flat).
