50W isn't enough.
It's 4ohm stable. That doesn't mean it has more than 50W. It's 50W.
The Pioneer is rated @1k. If you look, it states it has 220W
total power. 220/7 = 31.43WPC. That will not cut it for HT, even though it will be used for 2-channel. Not a wise purchase.
Now, this Yamaha has a total of 400W. 400/7 = 57.14WPC. So, you can get 57W for all 7 channels. And, you get a claimed@1k 115W. Probably around 85W 20-20k. Not bad for the price. This isn't enough for HT(IMO), but it does have decent 2-channel power. And, it smokes the other two options.
http://www.onecall.com/ProductDetails.aspx?id=31550