B) I am getting a new SONY head unit (don't bother suggesting other brands, I've made up my mind), a
CDX 8805X will soon be in my e36. This mp3 deck has 22 watts RMS x 4 channels, a marked improvement over my '92 325i's oem cassette deck with 5 watts rms x 2 (at 10% distortion?).

Is it time to upgrade to better speaker wire, maybe even Oxygen Free wire?
When I upgrade to a 10 inch Sub, what should I use then since I will wire that from scratch through a 150 - 300 watt mono amp.
I am building this system for clean, flat response from 20 - 20,000; not eye and ear bleeding bass that will surely crack my chassis's spot welds. I like listen to a wide range of music when I drive the bimmer; from Floyd, to Ozzy, to Crystal Method, to Ambient / Drone.

I am an ex-Radio DJ that is used to professional quality studio monitor speakers, so which brands of speakers with excellent frequency response do you suggest for my e36?