I use Nautilus 804 in almost the exact same system as you, except i use a Proceed AVP-S and quite different cables (see my system if you like)
This setup sounds fantastic on both home theater and 2 channel stereo. I would look at replacing the processor and taking a look at your cables which made a Huge difference.
You only listed your speaker cables with no mention of your other cables, but they can make more difference than the equipment itself if there is a truly weak link, so if you want that is the less expensive fix.
I have been playing around alot with power cords recently and the difference is HUGE. Soundstage presentation and depth are increased more than you would ever believe.
I never could figure out why everyone was buying all these expensive power cords,, now i know.
I hope this helps!
Greg