5 Channel Amp, Sub Channel Dead?

This weekend my subs stopped hitting...no bass at all. Yet the speakers all work fine. Amp isn't in protect mode or anything like that. I used a DMM to check the ohms from the subs (1.5 ohms) and removed the subs to verify everything was good...which it is. Therefore my question is...what's most likely to be the issue - the head unit (JVC KWAV-71BT) or the amp? JVC KWAV-71BT JBL GTO939 (6x9/Front) JBL GX963 (6x9/Rear) American Bass SQ 3.5 (3.5"/Dash) 2 - 12" SoundQubed HDS3.1 (1,200w RMS ea.) Hifonics BRX 5000.5 (5 channel amp) Knuconceptz Wiring/SkyHigh (0 gauge, 4 gauge speaker wire, RCA, etc.) Stinger 200 Amp Circuit Breaker Stinger 100 Amp Circuit Breaker Knuconceptz 300a InLine fuse Kinetik HC600 - 600 Watt 12 Volt Power Cell Battery PS - System was working fine for the last 6 months or so.

If the other speakers are working, the HU is ok. So likely it is the Hifonics amp.

That was my 1st thought, but just now (during my lunch break) I used my DMM to check my RCA cables. I disconnected them from the amp and unit. I set the DMM to check continuity and tested on connection - received continuity between Pin/Pin & Outer Shell/Outer Shell (but it took awhile to finally get it). Tested the other RCA and received continuity from Pin/Pin, yet nada between Outer Shell/Outer Shell. So if I'm lucky, the issue will be my RCA cable. Thanks for the reply!

Connect the front or rear RCA into the sub input of the amplifier, does it play at all?   Check every fuse on the amplifier.

UPDATE: After replacing the RCA, I still had the same problem. I set the amp to 3 channel mode (instead of 5) and it came back to life, but wasn't hitting nearly as hard as before. I replaced the HU w/Kenwood and - Problem solved. Therefore something went wrong in regards to the JVC HU and it's subwoofer connection. Thanks to all those who responded!