I pulled the radio to get to the Scosche SLC-4 converter and played around with those setting screws a bit. Doing that and re-setting the low pass filter along with the high pass filter helped a bit, it doesn't distort much anymore. I swapped out the rear 5x7 JBL paper cone speakers with
Infinity Kappa 6x8 speakers. I only need to do the doors but I don't want to mess too much with removing the door panels. I removed the driver's side speaker grille and I could see the speaker behind it, but there's just no way to wiggle the speaker through there. I'll just have to tackle the door panel removal. I did it a couple of times in my 97 Thunderbird but I never liked how they "fit" when I put them back on. Hopefully it'll be better on the Mark!
There is a better sound coming from the back since I removed the stock JBL speakers. These Infinity speakers are great! Thanks for your help and replies!