Recommendations for front/rear door speakers..

[edco]

Recently, I had a JL stealth sub and amp installed by the world famous Mobile-Addictions in Staten Island, in my 06 X5. Now the fronts sound "OK" but I thought I'd get some feedback on replacements for the doors, ie, do I do all FOUR doors, just the fronts (rarely have any rear seat passengers). I am not looking for BANGING bass you hear from 6 cars back, (I am 55 years old! ), but better highs and mid-range.

Also (this may be an urban legend), I have been told that since I use Bluetooth for my phone, I need to make sure that the fronts have dual coils (?) so that the audio from the phone comes through..I do not know if that's true.. I like Hertz. James at M-A had them installed in another car and they sounded great but candidly a bit over my budget if I do all 4 doors.

SO, for say $6-800, what could I do that would sound significantly and noticeably better, without breaking my bank (I am retired so I gotta watch the coins).

Would you do all 4 doors?
Would you replace the stock tweeters?
What size amp ? Brand ?
Speakers.. brand..? (why?)
I know that one can spend their entire disposable income on speakers and amps and such, but unless I want to live in a shelter for the homeless, my lovely bride ain't gonna let that happen..

Thank you for your advice, and of course, my best wishes for a Happy, Healthy and especially PROSPEROUS 2010 !

Ed in NJ