Head unit is mainly about the digital processor and power of amp inside. As I know, Alpine and Kenwood have the best power of AMP so you don't need to have additional amp to run your good spearks (about 45 or 50 RMS per channel). Those cheap chinese head unit you can found on ebay may only have about 15w RMS per chanel which is ok for factory speakers. You always pay what you want to get. I have apline on my Mazda 3 is great. After I got my aurion, I think Apline is bit too expensive so I went for kenwood. pioneer is good as well. Kenwood is better on quality of sound but pioneer is better on control (like ipod, iphone interface) better touch screen etc. Depend on what you want mate.