it says 6 ohms (compatible with 8 ohm receivers) on the product spec page.

are there any problems/downfalls with using an 8 ohm receiver with these 6 ohm speakers?

I am purchasing the yamaha rx-v661
info found here:

http://www.crutchfield.com/S-F4DjsrtkNQE/cgi-bin/ProdView.asp?g=10420&I=022RXV661

There is really not. The 6 Ohm rating is close enough to 8 that you should be fine. Yamaha has a good reputation here.

No problem. Hook it up and have fun.

Most receivers are 8 ohm default, and can also drive 8 ohm, 6 ohm, and 4 ohm speakers all at the same time. Just enjoy.