02 XLE V6, I've learned quite a bit over the past few days and unfortunately gdanaher's above post (while probably a little sarcastic) turns out to be right.... at least in the 2007 with the JBL Premium option.
With the OEM Headunit and the JBL "Premium" amp/speakers, there simply appears to be too much EQ-ing/crossover gaming going on with the Premium setup to swap in 'generic' aftermarket speakers. Unless you take out both the Headunit and the JBL amp (which I don't want to do because of the hybrid readouts in the headunit), the speaker replacements create a worse sound as JBL appears to be EQ-ing their amp channel to compensate for the cone characteristics of the paper woofers, for instance.
The "Distributed Subwoofer System" in particular must be expecting cone excursion/delays/abnormal frequency responses that the Infinity's, at least, didn't have. The end result was a bass that can best be described as out of phase. The end result was loud, didn't distort, but the sound never sounded in phase (and I switched the speaker leads just to make sure I wasn't accidentally out of phase with the wiring). And the general EQ of the sound was all effed up.
I ended up putting the oem dash tweeters (which are really mids too) and oem rear deck speakers back and only ended up keeping the Infinity 6x9's in the door. Having the Infinity's still in the door did tighten up the rumble a little bit, but probably not worth the money/effort given the small boost they give to the end resulting sound.
MAP1234, the Infinity's were 2 ohms which, according to another post I read, was what the OEM JBL amp/speakers were setup to do. (Can anyone confirm that?)
Morale of the story, for me at least, is that unless you want to take out the headunit _and_ amp of the JBL Premium configs, don't bother upgrading the speakers. ....unless, of course, anyone knows of a aftermarket headunit that will handle Toyota's hybrid readouts