I never post here, but I thought I saw an old friend.
Max-
Get a good stereo speaker pair and receiver/amp first. You can build the HT around it.
If you get the Polks, I would get a Receiver or amp that will be better able to drive them. The Pioneer might drive them, but I'd hate to have to listen to it. . .especially with crappy MP3's. I'm sure there's a workaround for your USB issue, so you can go with Den, Oink, HK, Rotel, B&K etc. I drove a pair of Polks' higher line with the HK, and it worked out well. You'd have an easier go of it with those 7's which are 8 ohm, instead of 4.