Or you could get an amp that has high and low level inputs. I still have a few bazooka 2150 amps that have 120 watts RMS per channel (90 to your door). With an amp like that, you could run speaker level inputs to it, or rca's if you upgrade later. If you run speaker level inputs from the front, I reccomend using a shielded cable.
There are many amps on the market like this. If you are wanting this to hook up subs, make sure it has a low pass filter (again, like the 2150) so that you are not sending highs to the sub. Also, you really should hook up the sub to the front speakers. This will let you keep your imaging up front so that you dont have to turn up the rears to get the low's.