I'm getting ready to install a 12 sub and mono amp in my GTI. It has the factory Dynaudio 300w system which is great. I don't want to rip that or the Nav unit out. Just looking to add some bass. With that being said what would be a good LOC to use for a mono sub amp? (ZX400.1) I don't want a cheapy $20 unit but do I really need something with summing and a bunch of other bells and whistles if it is only for the low end? (below ~140hz)