Here is another alternative. Get a 2 channel amp for the sub.
Sony Amp It's not pretty but I have heard nothing but good reviews... Add a 4 channel amp down the road. Use the built in amp from the HU temporarily and get the crystals for all 4 doors.
It'll probably cost about the same as the audiobahns with a 4-channel amp, and who knows, you may be perfectly content with the output of your HU's amp.
