subwoofer only hums how can i diagnose?

About a month ago, I had to take my audio system out of my car while some work was done on it (everything worked flawlessly at the time). When I recently reinstalled the system recently, exactly as it had been originally, the subs would not hit with the music but instead have been simply humming. I've double and triple checked all the wiring and confirmed that the grounds are solid. I would like to test that the signal I am feeding the system is good, but am unsure if my envisioned method is safe. I have a LC6i LOC that receives speaker level inputs from my factory headunit. Is it possible to hack up a pair of cheap headphones and drive music from a 3.5mm jack, using the cut wires from the headphone speakers as speaker-level inputs? If not, what kind of audio device besides a headunit could I use to "bench test" my system?

An amplified computer speaker done in the same manner as you were going to do the headphone will work great. And if you do not have a sacrificial computer speaker, you can purchase an 1/8 inch stereo extension cable from Radio Shack, you can cut that cable in half and use those wires.

DMM to check for voltage DC with system on to the RCA out puts that go to the Amp. Center positive outside negitive if no voltage then converter is suspect. RCA line voltage is usually around 2 volts, some are higher.

for all that it might just be easier to pull the radio out and just jump another set of RCA's to the amp. does the amp still humm if you unplug the rca from the amp?

Volt meter set to AC volts.