I think it's great you're trying to get him a nice present for his car, which from what you say is quite important to him. You aren't really into cars, but because it's something he enjoys, you're trying to research it a bit, which is awesome. Going to a store like Best Buy for face-to-face communication is well and good, but you'll be limited to the opinions of items they actually sell. Finding a dealer who will give you the best advice for your situation rather than the best advice on how to sell you a product they have will be tough. I'm not sure if it's just the Best Buy type stores by me, but another reason to avoid a big chain is that half the time the people there are morons.
I would say you're probably better off just giving him money towards it, and throw in that thing that us guys only get on special occasions; I guarantee you he'd be just as happy as if you'd bought the speakers for him