The ones no_escape suggests do have better frequency response than the Alpine SPR-69C you like. Although without knowing the specs to the current JBL's who's to say what aftermarket speaker will be similar or better in sound. Also, frequency response quoted by manufacturers are just a range, what highs and lows lie between that range is the big difference so don't go by Hz to kHz data alone. Some speaker manufacturers show the actual wave graphically-- if it is flat it will reproduce all sounds in-between the sounds exactly, if it slopes up or down or is wavy as hell in-between the range then it will not treat all sounds the same. But like I said, don't shop by frequency response alone.
If all that spec data crunching is giving you a headache just try
"Black Bob's Great Midwestern Ghetto Speaker Test":
1) Get a wad of 20 singles
2) Put a twenty-dollar bill over said singles
3) Hold money folded longways with the $20 showing first
4) Walk into a stereo store with a speaker demo display and find a salesguy
5) Small talk him about speakers while keeping your wad clearly in his eyesight
6) While talking, point to things using the fisted money-grip as a pointer
He'll let you crank all the speaker displays you want. If the salesguy tires of you cranking every single speaker on the display or feels your "shopping" is disturbing/scaring the other customers put the wad up to your chin and say "hmmmm"--that'll give you an extra 15 minutes. And if you don't like what you hear
or hear something that you like that you can get cheaper elsewhere show him that you only got $40 as you're strolling out!
