It's a relatively small enclosed space so 19 speakers or slightly more than 19 speakers aren't going to make that much difference. The cheapest low-power stereo with a single paper one-way speaker in each door is going to sound the worst and it gets better from there. But once you go to high quality two or even three-way in each door with a head unit that can power them properly, you're doing pretty good. After that, you get a small bump from separates and by lifting the tweeters closer to ear level and/or pointed better at the listening position, but it's a small bump and again in the small space you can just tweak the bass and treble a little bit. A sub and a bunch more speakers might give you a little bit fuller sound or whatever, but you're gilding the lily.