The sub is powered, its impedance doesn't matter.
RE: sirius internet radio - Do you have a source or does the reciever need to have networking and a frontend for it? Otherwise just about anything will meet your needs, starting with say the Yamaha RX-V371, or similar models from denon etc. I would avoid the slimline Marantz models, they seem to have heat issues.
There is a guy on ebay that sells b-stock RX-V367 for even cheaper, now that newegg has sold through all of theirs.
Basically you can throw a wide net if you only need 4 hdmi and no audyssey. Those speakers are pretty efficient.