Personally, I would just buy a head unit for cheap on CL (just enough to get proper RCA outs and possibly USB input)
I mean, $20-$50 is not much if you think about it.
A line out converter is how much? Money that you could put towards a head unit.....
Best buy/frys and others do financing as well...which makes it a lot cheaper.
Alternatively, you can just hook up an mp3 player directly to the amps...this only works if you have both amps near each other and you have highs and lows on amps, otherwise, I don't see the point of putting just the sub on it...