Get some 10g wire from www.bluejeancables.com. It's cheap and excellent cable. But your problem is that you are using car speakers for sound. Car speakers are made for an enclosed space and will sound like crap in an open area. Believe me, I had some expensive Proton car speakers years ago that I enclosed in a speaker box and they sounded like crap. But installed in the car they were fantastic.

You would be better off buying a cheap pair of bookshelf speakers from BB or CC. They have a few speakers for $50pr that would be perfect for a garage.