Audio loses highs and definition after high volume

Have a car in that is twisting me up in knots. Sounds good for a bit but after a little while (5-10 minutes) of higher/high volume, it's like someone engages a low pass filter and everything just goes muddy. Kicker CS components in the doors, Pioneer "smart" head unit, Hifonics BXX1600.4 out back.

I really don't want to just start parts swapping and crossing my fingers but my suspicion is the amp is very low quality/failing. Any ideas?