Pioneer's normally show ground loop issues more than any other h/u's. A few things you can try are.
1) Better grounds. Amp, h/u, and factory subaru grounds.
2) Make sure rca's and speaker wire are complete opposite side as power.
3) ground your rca's to radio ground/ amp ground. (rap copper wire around all 4 rca's, then ground to amp on one end, h/u on other end.)
You are getting a "ground loop" because your dealing with different ground paths between a common system..