Please help

[FFCEPJB]

I am going to try to explain this the best I can.  I have a 2019 Ford Mustang GT PP1.  It has the upgraded stereo but nothing is stock in it besides the head unit.  I have Focal K2 3ways in the front, Focal coaxials in the rear.  These are being ran on a DD Audio D4.75.  I have two 12" Kicker CompRT's being ran on a Rockford Fosgate P1000X1bd.  I have a bass knob up front on the dash to control the bass levels.  I also have a DSR1 that is being used.  Here is my problem......I sometimes get feed back through the speakers (gets louder with throttle).  This is intermitted and I have seemed to trace it to the RCA cables going into the DD Audio amp. The second thing that is happening is when the bass is turned up (about 60% on knob control) and it hits the interior speakers make a popping/static sound along with the bass hitting.  If I turn the bass down (0-10%) I can still hear a little bass but the popping/static sound isn't heard through the interior speakers.  I wouldn't think that the sub bass would have any effect on the interior speakers but it does, 100%.  I have used PAC and Stinger RCA noise isolators with no help on either problem.  I have changed the crossovers units themselves and made sure all wiring was correct.  I have checked and secured the grounds.  Is it just the RCA's?  I have had my car in the stereo shop twice and it seems fine when I leave but starts making the noise later on down the road.  Stereo shop I use is 3 hours away from my house in PHX and I have no one local in my city.  I can try to change the RCA's myself but dont want to get into my daily driver without knowing this could/should be the fix.  Is there anything else I can try myself.  The next thing the stereo shop said was that I need to rewire the whole car, which may be the best if it will really work and be done with this.  Its very frustrating to have a new car and stereo without using it to its full potential.   Any help anyone can give would be awesome.  Thank you all for your time.

[Dafaseles]

9 times out of 10, when you get feedback that changes pitch and/ or volume with the throttle, it's a bad grounding spot or not enough grounding for the system. Now that could be for any piece of equipment in the chain. 

My suggestion is to get your hands on an RCA to whatever your phone jack is, and try taking your head unit out of the equation, see if it stops. If it doesn't, start working your way down the chain. 

Also, if you have a digital multimeter, you can actual check your grounding spot(s) to see of they're sufficient for your electrical current. There are videos on YouTube that explain in detail how to do it. 

Now the speakers popping when the bass hits, that either sounds like a loose connection in the chain to your speakers somewhere, that jiggles when the bass kicks hard, or somehow you have your bass knob wired into your speakers signal somehow and your playing too low of notes over your mids.