Checking in again. Been playing with this tone generator, using bluetooth on the iPhone:
www.szynalski.com
I've used this tool for testing crap speakers vs good ones, and for setting crossover Hz from subs to mains on my home systems. I was curious how the base RTL speakers would respond to this. Just did an "ear check" FWIW, no fancy scopes.
Subwoofer: This operates from 25-220 Hz, and is really inconsistent across that range. Either putting the ear on the back seat or sitting in the driver seat, frequencies drop out or get muffled significantly across that range. They completely disappear from 140-150 Hz, come on strong again from 170-200 and fade out >200. In music I notice how certain bass tones drop out while driving. Opinion: I have two subs in the house and this RTL one acts like my cheap one but worse. Terrible!
Door speakers: These start responding at ~50 Hz and run up to about 12,000. They give fairly consistent sound. Except for how the fader is biased towards the front, I don't hear a difference between fronts vs backs, and the speakers do have the same part #s online. Opinion: honestly not terrible for what they are - basic cones, probably paper.
Tweeters: These start responding at >3000 Hz, but they are plain harsh! Nastiest tweeters I've heard in a long time! I understand there is a bass-blocker cap in-line here.
I used to mess with this stuff from the early 90s until about 10 years ago. I think I'm going to get into it again, starting with a speaker upgrade. Going to do the tweeters, sub & door speakers in that order, and check a few things along the way. I don't see how this can hurt sound quality with speakers that are rated to work with modest W RMS. If it needs an amp, then that will come afterwards. I'll report back my findings to this thread as I go.