You know what makes and breaks most shops I will tell your right now.
I am really into stereo industry and really the huge aftermarket industry. My dad runs his own business. Well 2, onces all garden and power equipment and the other is carson trailers and polaris atv's.
Its really important for your shop to be clean, to look very nice inside and out, you have to pay attention to everything. If you want to make a million bucks, then make your shop look like a million bucks. If your showroom looks good, people are willing to stay longer and possibly buy more. Making sure its the right temperature is also a big hit.
Also make your shop look good too, people want to see where the work gets done, if it looks like hell, then your going to lose a lot of your high end customers.
Keep everything organized, and priced clearly. try to get a good software for doing your inventory,sales, and stuff like that. Make sure your phones have a professional sounding recording or any answerings.
Also keep a good log of your customers vehicles. It's really important because people want to see what you have done. So taking tons of pictures is a pain in the ***, but really makes your more credible to the unknowing customer. You can even make displays of some kind on walls with the pictures, or just put them in binders on display.
Your brand selection looks good. heres the thing though. When choosing some companies its really hard to compete. Because you have the circuit city/best buy/frys. and compaines like that. While they only handle lower end products, they buy them at like half the cost. so choosing what models is key. Not only brand selection but make you research what your local big shops have before you bring in a whole bunch of one deck that you cant sell for a reasonable price.
Also back up your products, offer a good service, warranty. A lot of big companies wont let you take things back as easily, Make your customers know that if they have any problems that they are able to come back or call.
thats all i can think of right now.