um, do this man, go to like auto zone and buy like a 15 or 20 dollar Digital Multi Meter. and you set it to the ohms section, most likely your mm will be only capable of down to 20 or so. you hook it up to the box. If you have a single 4 ohm voice coil sub, the sub should read anywhere from 3.5 up to 4.3 average. If its a dual 4 ohm sub you could see anywhere from 1.7 up to 2.4 or 7.6 to 8.3 ohms. Depending on the way its wired.

You can also check the rca's. by taking the two little probes putting one probe on the center spike of the rca's and the second probe on the sheet that looks like a guide to the outside sheild of the same rca. and put your mm on voltage. If you are getting a 0 voltage then your rca's could be bad.

if the amp cuts on, then your problem is in the signal transfer ie, rca outputs on the hu are blown, or the rca's are bad, or the speaker wire from the amp to the sub box isn't connected or bad, or the wire inside the box is bad/blown, Or the subs themselves could be blown.

A digital Multi-meter is a must have. I have a buddy who lives down in columbus who is An Audio guru... send me a pm, I'll transfer that info to him and have him call you. He may charge you like 15 bucks to trouble shoot or he may have you roll into his/his buddies shop idk but he is local to you, and very well known in the car audio world!