I felt a big difference when I switched from 2.7V(panasonic) to 8V(eclipse). The music got a lot crispier and better sounding over all, but that might just be going from crappy panasonic to eclipse. I've had alpine before and it sounded pretty damn good. I'd say better then Stock/Clarion/Panasonic. But it didn't come anywhere close to Eclipse

. So like I said if you are not addicted to extremely good sound systems and you are not going to compete in SQ class. Pretty much any alpine will do the job. And if u wanna go SPL with low voltage pre-out deck like the one we are talking about in this thread. You can just buy
AudioControl OverDrive, this thing will bring the voltage up to 12V. There are some drawbacks if using this unit but only if u compete in SQ. This unit can be had for about $100 (one of my friends had this unit before he upgraded to Eclipse deck and it worked pretty good).
TimeAlighment - is an option in high end decks that works like sound delay. Here's a lil explanation for ya ppl. The perfect listening position in Home Sound system is right in the middle of two speakers because stage and everything else meets at that point. The thing with cars is that driver sits closer to one speaker which screws up what sounds come at which time. So thats where time alighment kick's in, it delays the speaker closer to u by millisecond's and makes it so the sound from to speakers arive at your ears at the same time.
Equalizer is another good things to have, and I am not talking about equalizers that mid-decks come with. I am talking about Parametric EQ's and 260point band EQ's. Because our cars are made from different materials, some like carpet and seats absorb frequencies and dash, plastic reflects it. Which amplifies or kills some frequencies, so with a use of EQ and some really good tuning all the frequencies can be played.
PS: People that say "hey I bought a clarion and it sounds just fine" or "I just put in an Alpine 9853 and it's amazing for $240, even on my stock speakers before my components get here." probably never sat in a car with a good system. I'll be coming to NY celica meet this summer and I am pretty sure my car will be done by then, I'll let a couple of people in to listen and see whats the different between system and a good system.