As outlined on the first page, I want to demonstrate the value of changing the BMW speakers while leaving the electronics intact. To make the point, I'm using relatively cheap drivers - later, after we know this is going in the right direction, we can always move upmarket if needed.
Here's the complete analysis, speaker-by-speaker...
Front Door Pillar Tweeters
The stock BMW tweeter has a 1.00" body OD, 0.50" depth, 8 ohm voice coil. They're quite cheap, soft poly dome / small voice coil / lightweight magnet. The sound is just a gentle mush and they can really use an upgrade. I contemplated using "car" tweeters such as Infinity / Polk / Boston, but finally settled on some Audax drivers from Parts Express. Given the way retail merchandising works, it can be far more cost effective to buy loose OEM parts than boxed, branded, retail sets. I estimate the Audax tweeters to be in the same realm as the mid-range Infinity / Polk / Boston gear.
The Audax tweeters have the same 8 ohm rating as the stock BMW parts, so they're compatible with the existing Alpine amp. By removing the outer flange from the tweeter, I find they fit snugly into the BMW plastic carrier that bolts to the door pillar. This Audax is an excellent fit for the 325i / 330i door. It's bigger than the stock BMW part, as big as it could possibly be, but it still fits nicely.
The Audax tweeter uses a textile dome and larger voice coil. Sound is significantly better than the old BMW part, and a bit louder. The highs are "lifted up" above the previous mish-mash.
Front Door Midrange
2.50" body OD, 1.25" depth, 8 ohms, paper cone with rolled cloth surround. This turns out to be an OK driver, compared to other, weaker system parts.
I compared it to a Peerless poly dome midrange, by switching the connections and listening to CD's and radio. There was little difference between the two, especially when considering all the other things going on inside the car. For now I've decided not to change out these midranges.