I've got a 98 sixxer with the regular stereo system (not mach460), and while it doesn't sound bad, its not great either. I'm wanting to replace the front and rear speakers (only got 4 I believe) to get some better quality, while still using the stock head unit. I don't want someone to see my head unit and go nuts and break into my car, whereas just new speakers are not nearly as noticeable. What sizes do I need, and how much would I spend about on something like this?