I plan on purchasing a new head unit for my '07 Tacoma Double Cab. After a lot of searching, mostly on Crutchfield, I have decided to go with the Sony CDX-GT610Ui and would also like to add the Sony XM-4S amp. I have Polk db690s in the front and Polk db650s in the back. My question is how does the speaker level input set up work? And how do you get