Speedeamon15

I have recently purchased a 01 gt with a mach 460 system and a custom sony head unit. I was told that the mach 460 system has subs in the doors and is a pretty impressive system. I am affraid that when they took out the factory head unit and put the sony one in they dident hook up the subs or do it right. The stock amps are still thier but the custom head unit has a sub level instead of a bass leval. So what i need to know is if anyone is familiar with this problem and if i can hook the factory amps and subs up to the sony head unit. My next step is 2 10"S and a big amp if this will not work out and i really dont have the money for it right now all help would be greatly aprechiated thanx