Hi,
I'm not familiar with this kind of sound enhancing devices like the Alpine PXE-H650, but according the product description it tries to fix the standard audio signal by doing some complex digital sound processing. I guess the PXE can fix some issues and improve maximum the sound level of the basic stereo unit. Off course only when you use good amplifiers and speakers. But clean sound reproduction will still be an issue.
So spending a lot of money will not return the quality you should expect of it.
I would suggest starting with a subwoofer system to get a nice bass sound. But that isn't easy. A separate subwoofer and amplifier (<80 Hz) is required. This amplifier should accept hi-level input signals and have a low pass filter. This amp should be connected on the front speaker signal. This is a must because the rear speaker signal is seriously high pass filtered, not something you want to do on your sub woofer signal. This will give the feeling the system already sounds louder. But top down will still be an issue.
About the basic stereo's equalizing; the sort experience I had with the unit was that the front output was not that heavy equalized. So it could be used as source signal to feed some amplifiers. So you could skip the PXE and try this setup. I would NOT use the rear output signal to feed mid and high range amplifiers.
Than we have the speaker story. I have replaced the rear speakers with full rangers.
I personally think that the foot well speakers are good enough for the job. Just use an extra amplifier and high pass (100 Hz) the signal, than they can play much louder than before.
I'm still busy with my audio system, I guess you can get some info from this threads:
http://www.z4-forum.com/forum/viewto...p?f=23&t=11335
Success, EdP