I'm going to have to agree with Jo-psyko an 8" powered audiobahn sub is just a waist of money unless you installing it in the kick well.

I would buy the head unit you need for your USB connectivity and take the rest of your money and invest in front speakers with dynamatt or if your front speakers can last a bit longer I would add a larger/higher power sub and put a low pass filter on the front speakers (you might want to add that to your head unit spec) or you can add a coil to each speaker.

The sub option is the one that will make the most difference and then save up for the front speakers it’s entirely down to your current system.