Unfortunately, lowering your crossover setting won't deepen the response of the satellites, but it could create a gap between the subwoofer's high point and the satellites' physically limited -3dB low point. I'd stick with the Denon's recommended crossover point. If you need your mains to play deeper at a meaningful volume, you'll have to replace them with bigger speakers.
Surround speakers behind the front speakers... Well, they won't really do what surround speakers are intended to do back there. If running them to the sides of your primary listening position (or behind) isn't possible, I'd probably sooner just leave them unplugged than keep them active behind the main L+R. But do what you think sounds best.
If you're happy with the sound, then I won't argue with your ears. I don't think you're in any danger of releasing the magic smoke from any of your components, in any case.