[Big Will]

You could simply upgrade the car's main speakers (front and rear) to Alpine, Pioneer, Kenwood, JL etc. and add an amp/sub in the trunk. It's pricey, but Volvo does make a perfect fit for the trunk. That would retain the factory headunit but should give you better sound.
Hope that helped.

 

[Big Will]

So there are no amp/sub outlets on the stock radio HU? That's too bad if it's true though like d_man says you can still add them to the rear speakers