Has anyone added an amp and sub to a aftermarket HU, but kept the stock speakers intact? I have done this on all my previous VWs, but I don't know if Subaru's stock speakers are good enough.

I'm not looking for audiophile quality here, just a little more 'oomph' if you will. The sub I would be adding is a 12" Polk Momo powered by a Sony Xplod amp. The car is an 07 STi.

 

long as you have RCAs out of your deck.. but the sub might overpower the stock speakers.

 

stock suck, get rid of them, dynamat the openings, get some component 6.5 front, rear if you want 4.5 coax. with an amp, 4 channel. best route to go, you will have less head aches later. trust me. i know. rollin natural sucks.
you will blow those stockl speakers, they are paper junk.