I have been shopping for an HTS (my first one) and have been doing research on what to buy that will handle a pair of vintage Polk Audio SD II 4 ohm speakers that I acquired. After reading in various Denon threads regarding what the Denons can handle I bought the AVR 791 (just yesterday). I got the impression that if you didn't push too hard, most receivers (like Denons) would handle a 4 ohm load, and that they don't list it for < 6 ohms because doing so would require (a higher?) UL certification. I also bought a Polk CS-10 center 8 ohm to go with the set up. I have a pair of 16 ohms that I am thinking of hooking up for the surround rears, but have not done that yet. I have not bought a sub yet or chosen the other two speakers to complete the 7.1.
So, I have an AVR 791 with two 4 ohm Polks for the front R + L and an 8 ohm Polk for the center only, all connected with 12 gage bare speaker wire (until I get some banana connectors). I have not experienced any difficulties with the 791 shutting down or giving me any bad audio at all. The top of the unit gets slightly warm so far, but that is it. I have it connected to the HDMI sat/cab box, HDMI PS3 (also component to my Wii), HDMI XBox, HDMI BD LG blue ray---> HDMI out going to my RCA 1080p TV.
I have put it through the paces as it stands so far by playing all the game systems and TV and blue ray dvd (Batman's The Dark Knight and Cars for testing) as well as music CDs, ipod, and USB mp3s, and the radio tuner all at various volumes--including ~ -15 or so just to see how it handles. That was plenty loud enough for me and my kids, and the regular volume for us seems to be between -30 and -25, occasionally going up to -20 or so (but probably not too often). As I said, the unit so far has only gotten slightly warm on top (after several continous hours of testing).
We are still experimenting and trying to decifer the manual to see what other setting there are (such as tuner and audio presets, etc). I am always looking for helpful advice--I too, am a noob at Home Theater (on a budget). Any advice or thoughts on speakers for the SW, SL, SR, and/or other speaker combinations would be appreciated.