My friends are lame- no one has a 6XS lol
Puzzling as it seems, I pulled the HU today:
1) moved the ground and same issue
2) ran a spare RCA cable outside to the 6XS and all is well.
Turns out the front to back RCAs were introducing noise somehow. I was careful not to put power and signal closeby, but I figure (in trying to be neat) I may have routed cables too close a factory harness somewhere. Fresh RCAs will go in tomorrow via another route and then I can proceed to tidy the install. Thanks to everyone for your suggestions and input.
By the way, this flow chart from Crutchfield is a good method to follow to help isolate system noise:
https://images.crutchfieldonline.com/learningcenter/articles/417/NoiseSuppress.pdfNot sure if it was shared before but, even though I tried everything before getting the flow chart, I'll still keep as reference.