How to diagnose issue with sub cutting out?

Hi everyone,

My nephew had a sub, amp, and head unit installed by a local car audio shop at the first of the year. His sub quit working completely a few months ago and he asked if I would look at it. I had limited time and no multi-meter, but I could see that the amp was turning on without going to protect and all wiring looked to be solid and done correctly. I checked the wiring to the sub as well and it was fine - with no signs of being blown when I inspected it visually.

It's a pretty simple setup really. A small 600w (peak?) amp with matching sub and prefab box. The amp is two channel but has a bridge mode, so the sub is bridged for 600w to the 10in sub. Anyways, I told him that the only other thing I could think of to check was his RCAs, as they could have came loose or the audio out on the HU was having issues. He skyped me a few days later and I walked him through the process of taking his HU out and checking, and it turns out that the RCAs were indeed disconnected. The RCAs were connected using a 2 RCA female to 1 RCA Phono male Splitter Y Adapter. The single male was plugged into the sub out. After this it was working fine.

He called me a week later with a new problem. The subwoofer will cut out on heavier bass hits when his volume goes past 20 on the HU (max 40). I messed with the gain and bass boost and managed to get him to about volume 27 with only the occasional cut out. The best I can describe it is on a heavy bass hit, the subwoofer sounds like its playing on volume 3 while the rest of the speakers are on volume 27. It usually only does this for the single bass hit and then plays normally. The higher he goes past 27, the more frequent the cutting.

For now, he is keeping the stereo on low volume. I am going to take a look next week and I plan to have my multimeter this time so I can set the gain properly on the amp and get rid of the bass boost. However, I am looking for other things to check that could be causing this issue. He mentioned that it was "just as loud or louder" when he first got it installed and he never had the issue, so I'm thinking this goes further than setting the amp properly.

Can someone please provide me with instructions on how to check if the battery is properly supplying the amp with enough power, any any other guides you think would be useful to diagnose this issue? All of your advice is greatly appreciated.

Thanks!