Adding an amp with factory radio...need advise
 

07 Jetta and I have a 400 w 4 channel amp from my Mazda and wanted to get it installed with the factory head because at least at this point I don't think the factory unit isn't bad at all. Basic yes, but I like it compared to my previous vehicle's factory unit. Plus it looks nice in the car as is and looks cool illuminated at night.

Anyways, I know I have to get amp cables and an output converter which I know a basic one only runs 20-30 bucks. Basic gauge amp cables run 60-70 dollars at most stereo installation places. But this one place I tried going to wanted to charge 400 bucks for the installation. Sounds pretty high to me considering another place I took it too charged 300 for installing the same amp with the cables along with a cd player and 4 new speakers. This place was going to charge $175 dollars in labor which to me just for an amp is ridiculous.

My other question is even if I got a line output converter and want to eventually upgrade to also installing subs with another amp and a crossover wouldn't I need a 6 channel line output converter? Most of those I've seen are over 100 bucks. Perhaps if I do that then leaving the factory unit in it isn't worth it at that point? I really like the factory speakers which is amazing. I do think if I got some better EQ out of them from a unit then maybe it would be better anyways. I don't know the wattage of the factory unit either but it also can push the speakers better then any other factory unit I've ever had.