to get the mathmatical explanation of why biamplification is better, i think audiocontrol has said it best. look on their website at the PDF's they have, a LOT of great information. basically, the amp isn't trying to do an entire spectrum range and doesn't have to work as hard, making it more efficient, giving you more headroom, giving you less distortion, and giving you more control over the sound. i had a GT22 on mine for a while, and they sounded great. i got rid of my 5.1 setup and the GT20 i was using for my center channel no longer had any use, so i figured why not use that on the tweets. it made them sound like entirely different speakers. and by bi-amping them, it gives me more flexibility and tuneability. i try to keep a stock look in my car, so they are in the stock locations with the mid low in a very well deadened door, and the tweets in the sail panels. it gives me the ability to use the time alignment more efficiently by giving me seperate alignment for the mids and tweets independantly. helped the soundstage a ton as well.
the main difference you'll hear between the Z's from the pro's is they'll take way more power, have way smoother tweets... the pro's are more like lazerbeams; the midbass is more present, and when you start cranking it up they'l hold their linearity all the way up, as where the pro's will crap out and start losing quality after a certain point. looking back, they are definitely worth the extra money. if you're gonna do it, you might as well do it right the first time, and not wonder late down the road how much better it might sound had you sprung a couple bucks more for the Z's...