Thank you all for the suggestions and helpful advice. I have decided to get a decent mid-range AVR along with the 2 channel crown amp.
Just couple of points for you to consider:
a) The graph TLSGuy posted is for the diamond, not the Nautilus.
b) I could not find such a graph for the 802 Nautilus, but I think they would be just as tough to drive.
So if you want optimum performance and/or listen to levels reasonably loud, unless you sit very close to those big speakers, a mid range receiver may not be able to drive some power amps to sufficiency high output level.
Among the popular mid range receivers, the ones that have the best chance to drive just about any 200 to 400 W power amp to their output limits are Denon's, based on bench test measurements.
One last point, for Crown amps, you should pick one that has the lowest distortions and highest SNR. It would be a pity to use an amp that don't have specs good enough to guarantee transparency, to drive the 802 Nautilus.
If you can stretch your budget to above $700, a great amp for those speakers is the Hypex based Buckeye amps.
It is not only relatively cheap, but light and small too as you can see in the attached photo. That tiny one sitting on top of the gigantic one is 40% more powerful, and at just below rated output its distortion and noise is much lower as well.