Warning - long post ahead.

Car and system:

08 M6 Vert, no mods but stereo
Pioneer AVH-P8400
Harness from DDmods for steering wheel controls
Arc KS mini 125.4 under passenger footwell (modified by Arc for 10x crossover freq)
Arc KS600.2 in trunk
Polk MM6501 (each driver running off 1 channel of 125.4, crossover ~3500Hz)
JL Stealthbox 8" subs behind seats
Polk MM1040 subs in trunk
Each set of subs running off 1 channel of 600.2 in parallel for a 2Ohm load per channel
0/1 gauge power cable from battery to distribution block under passenger footwell
4 gauge power/ground cable to each amp
Ground cables to ground block, connected to ground point with 0/1 gauge cable

The problem:

The volume of midrange frequencies on the driver's side fluctuates randomly. The treble seems unaffected. The midrange speaker does not completely cut out, and the lower frequencies from the midrange speaker seem unaffected. There is no pop/crackle. The midrange frequencies (guitar, vocals, etc.) randomly drop in volume, then come back.

Usage:

I commute about 45 minutes each way, mostly interstate. If weather permits, the top is down and the stereo is loud.

Testing:

I checked all connections and verified they are solid.

Turning off the sub output and changing the high pass crossover on the deck has no effect on the frequency dropout. Using balance/fade to isolate the driver's side mid/tweet or the mid alone reduces, but does not eliminate the problem. The battery gauge on the dash never drops below 14V when the car is running.

I can't pinpoint when the problem started, but it was not there after the install.

Preliminary conclusion:

Possibly a heat-related problem with the 125.4. The crossover mod performed by Arc may be failing, causing the crossover frequency to drop from 3500Hz to some undetermined frequency, but I have no explanation why it would only affect the driver's side.

I considered that it may be a problem with the electrical system being unable to supply enough current consistently, but turning off the subs doesn't help, and the voltage reading never drops out of spec.

Questions:

Has anybody else experienced a problem like this? Any suggestions for a fix?

 

Any chance you have GLI between HU and amp? I had one doing similar strange things once, but it was mostly the high frequencies cutting out.

You can prob isolate it down to a specific component by swapping left/right wires. Start with RCA's going into amp. If problem moves it's HU or RCA. If stays on left, then put RCA's back and try swapping speaker wires left/right. If it moves, amp is bad, if it stays put wires back to normal and swap crossover/speakers left/right. Eventually you'll be down to one piece that you should be able to swap and predict when the problem will follow.

 

No GLI. I'll try switching RCA next.