only one sub playing at half power

I had my old system hooked up and it stopped working so I just assumed my amp had died because it was old.  I went to put my buddies system in my car and could only get 1 sub to come on and it was only getting half power.  So then I assumed I just needed to replace my RCA's.  I went ahead and bought a new amp, and installed it with my subs.  I ran a new remote and new RCA's.  However, I am still only getting power to one sub and its not getting full power.  I have checked my ground and it is fine as well.  I also hooked my RCA's and remote up to my friends headunit and had the same result.  Does anyone have any idea what the heck is going on here? :(