the 400.1 is a mono block sub amp. I was only running one ten as it is a 4 ohm dvc sub. The amp is rated at 250x1 rms @4ohm and 400x1 rms @ 2 ohm. Like I said I was only running the one 10 and had it wired to run at 400 watts @ 2 ohms. The extra sub was one I picked up a while back that a shop had in their display board. I bought it as an extra in case I needed it for something. I have never even hooked it up. I figured I would toss it in with the deal because I really have no use for it.