Hi Herb,
be interesting to hear how this goes for you, nice to have more power and a smooth running machine at the same time thats for sure.
I dont have a gpx, but i just sold my gp3500 to buy a fully modded gp3000, so i cant recommend anything to you. I'm yet to test mine out using a mono, though my plan is to lower the frequency and gain to level that removes most of ground noise. Have you tried dropping your factory gain down low in relation to the var gain. i saw a youtube clip just the other day, A guy called Nenad showing some evo coils 17x13 and 19 with factory gain on 1, which was still better than fine gold or enhance it seem to remove a most of the ground noise too. Good luck with it, hope you can get it running sweet.
Joeboy