Unplug the sub amp and try the Crunch amp on the subs. See if you can get anything... if you can, then its probably your amp or wiring to your amp imho.
If you unplug the sub amp and the problem persists, then it might be your RCA's, Head Unit, etc
When I mean unplug.....actually unplug it. Remove the 1/0 gauge from your battery so the car doesnt even think you have that amp anymore...so to speak.
You have another amp right there. Just remove the speaker wires, plug in your amp and try it that way bypassing that sub amp altogether.
If you want, you could also use the Crunch amps power/ground/RCA since you know those are working. Just use those in the sub amp. This could possibly test your power/ground runs to the sub amp and the RCA's.
I would still recommend getting a DMM though. I know you said you took it to a shop, but with a $20 DMM you could be in your driveway testing it instead of a shop. Gotta get the right tools!
Typically when I have an issue that I cannot resolve I will remove all the power at the battery. Reconnect 1 amp and 1 ground to 1 amp. Test it. If it works, move to the 2nd power/ground on the 2nd amp with the 1st amp removed. Test it. If it works, then hook both amps up. Basically try to narrow down and troubleshoot. If you know your Crunch amp is fine without the sub amp connected then that is a start. If you know that both amps are fine until you wire them BOTH up, that is another issue...etc..etc..etc
And of course keep looking/posting on the forum here with what you have done or what your results are. Write down what your doing so you dont forget. You could possibly have done troubleshooting method A,B, and D but you totally forgot you didnt do C...and that was the issue. I have skipped stuff or forgot and kicked myself later for it.