Possible amp/sub failing - Please Help!

[Reaper]

I have a Pioneer GM-D7500M amp and Infinity reference 12" SVC subwoofer. I have a new battery and alternator, and 12 gauge speaker wires going to the sub. I also have 8 gauge power wires going to the battery. I got RCA cables going from a 4 volt sub out on the deck. My system used to work great, but about a year ago the subwoofer started cutting out for a second during hard bass at high volumes. Decreasing the volume made it go away. The manual for my amp says there's a safety feature which shuts it off if it tries to put out too much power, or from playing at excessive volume. I checked all connections then researched how to properly set my amp with test tones. I adjusted my deck's EQ and my amp's gain and my system sounded even better. The amp also stopped cutting out. About a month later it started cutting out when playing music with a lot of bass. I readjusted my amp and lowered the gain again. The problem went away for a while, but I had to keep doing this. At this point my amp is set really low and it's not putting out enough volume, but it still cuts out. Also, the bass quality from the sub has gotten worse. Is this the amp failing, or the subwoofer? I don't want to replace one and find out that the other is the problem. I looked up the specs and as far as I can tell my amp puts out 400W at 2 ohm and 200W at 4 ohm. There are so many similar models of these subs so I'm not sure exactly which one I have, but I think mine is rated 300W at 4 ohm. Could an underpowered amp be the problem? It used to work ok but has gotten worse. Do I have both ruined at this point?

[Chris from Washington]

Hi There,
Welcome to the forum.

My guess is your amp is going into protect mode at those high volumes. Where is the gain currently set on your amp?
Just from experience whenever something like this happens, it's been because the gain was just set to high. If your deck has a 4volt out, your gain should be set pretty low.

I would just try setting it lower, and see if the problem keeps happening.
Does seem odd that it played for awhile that way, but try turning it down some more.

[Reaper]

That's the problem. I tried setting it down. It works for a while, then I have to set it lower and lower. It seems like it's getting worse over time. Right now I have it at the "normal" setting (about 2v) and the sub volume is way too low. I just get very mild bass with no thump. I usually have it between quarter and halfway up but now the amp cuts out at that setting. when I have the deck turned up with bass heavy music, it works for about a minute then cuts in and out. The sub plays a couple of beats then mutes a couple of beats. The volume on the deck is only about half way up. It worked fine when I first had it, with the gain just under half. At the current setting the amp doesn't cut out but I noticed the sub is starting to sound distorted for the past week. Would the amp cause the sub to sound bad if it was failing, or will I have to replace both the amp and sub at this point? I'm not sure if the sub is bad and causing the amp to cut out, or the amp is bad and making the sub sound terrible. The sub still plays really low notes ok but it's easier to hear the higher ones distort. I used to have my deck's crossover set at 125Hz but now I have to keep it under 100Hz or I hear more of the distorted bass.