I would just buy a 2 channel amp around 100 watts per channel and just run the deck power to the rears. Thats just my opinion. I think if you can stretch your budget a bit this would be an awsome amp. I had a pg amp before and thought it was a very solid amp.
http://www.cardomain.com/item/PHOTI5004

I think it may be a bit hard to find a good amp that does 50-75 x4 for that amount unless its used or you ebay it.