Sub and Amp for 07 VTX

Ok so my new boat has the base stereo package for the VTX, which is 4 speakers, a sony head unit, and no amp or sub.

I'm not a stereo junky, and don't want to drop a grand on these two components, but do want something to give my speakers a little more drive and a sub to round them out. Now later when I put pro60s in I will be looking for a higher end amp for them, but right now I just want a little more power to enjoy my music without breaking the bank anymore than I already did buying the boat.