I used to run a Honda factory HU with some amplified Polk components. I was able to tame the tweets with a combination of the HU Treble control and the -3dB switch on the Polk xover. BUT, I was never fully satisfied. Only a new headunit with EQ or a DSP is likely to fully scratch that itch in my opinion. The P660C's are excellent speakers (I run a set active now with a DEH-80PRS and love them) but -6dB may or may not satisfy you. Seems a shame to toss the Morel's on a gamble that may or may not pay off. For a lot less money, buy a pair of L-Pads from PartsExpress and experiment... partsexpress
If that doesn't satisfy, then consider a processor or new head unit. The PPI DSP88R isn't much more than you were going to spend on the P660C's