As a "part-time audiophile", I find the whole "900 watts" thing quite funny, to be honest.
Most home audio speakers have a sensitivity of about 90 db. Which means, when you use 1 (yes, "one") watt of power, sitting 1 meter away from the speaker, the speaker will emit a sound at 90 db. Which is quite loud, some people break out hearing protection at that point. To raise the volume by 3 db (most people can barely detect a 1 db change) to 93 db, would require doubling the wattage (so now you're at 2 watts). Raising that again by 3 db requires a double again (4 watts).
So 99.999% of all music listening is done at < 10 watts of power. Only under very short and extremely loud bursts (like a gunshot or explosion in a bluray movie) would you need to have more than (say) 25 watts of power.
Don't fall for the marketing; the number of watts is completely and 100% meaningless. It's the quality of the watts that matters.
Since most car audio is "class d" (instead of class a/b), you're not getting very high quality music anyway. And most of these setups are designed to emphasize the treble and bass, at the expense of the midrange (vocals) because that is pleasing (but ironically "less accurate") to most people. Add in other variables like road and wind noise, you're better off skipping the whole idea of high quality audio in a car, saving your money, and putting that money into a home system that actually sounds good.
I mean, I turn up the volume on my radio half way and the distortion and ear fatigue is so obvious. Throw in a bluetooth audio source, I have to crank the volume to 75% or more for classical music that actually has some dynamic range, and it becomes unbearable to listen to. I don't know why they even bother putting radios in these things, may as well just give everybody a free pair of ear buds to plug into their smartphone and call it a day.
