Never used their stuff except the 2XS a while back. From what I can gather, their stuff is really good. I suspect you're trying to integrate higher end stuff with a stock head unit. I read in a previous issue of Car Audio about a Mercedes Benz AMG E55 that used the EQL with the factory head unit (it controls most of the in car electronics). I think the EQL included a balanced line driver and EQ with speaker level inputs. I think JL also makes a similar product.
JL Audio makes the Cleansweep and I think Alpine just came out with a new line out box too. They're all expensive though and I'm not sure how much of an increase there is in performance and sound quality. I read the JL Audio is coming out with a cheaper version next year. The current CleanSweep costs over $300. But it has an Aux input which is cool if you want to add an ipod or something.