Why does my rockford fosgate have 2 inputs for the power and ground terminals on the amp?

I installed just a single power and ground lead, and listened to the amp. It sounded as if it was operating properly, but I wasn't getting a lot of output. I know how to install everything correctly, so my question is why did this amp not perform very well? Is it because I need the additional power and ground lead? My buddies alpine which is 300 watts less rms than this was plenty louder. BTW, this is an old school rockford 700s. 2 channel. 175rms by 2 @ 4ohm, 350 rms by 2 @ 2ohm. Thanks.