Originally Posted by Beej
explain to me the difference between a unit that has 2v and 4v. Why do all the high end units have 4v.
Not all the high end units have 4v. In fact, the latest McIntosh I saw has only 2v, and their stuff (while hideous, in my opinion) is considered to be really high end as far as sound quality. A lot of other nice units out there are 2v.
It's not as big of a deal as some make it out to be. I'm no expert, but from what I can tell it's all about matching your head unit to your amp gains. If your head unit has a higher voltage output, you can normally keep the gains lower on the amp in order to match up with the head unit. It shouldn't be so much of a difference that you'd have to crank your amp gains all the way up if you get a unit with 2v output.
Interestingly enough, there are some (older) amps out there that don't like too high of a voltage output from a head unit. For example, I have 4v output from my H701, and with my old PPI A600 I have the gain turned all the way down when set properly with a DMM. If I turn up the gain at all the voltage gets way too high in a hurry. If I used one of those units out there with 8v output, I might be in trouble. I've heard of other amps having similar issues with really high voltage head units, but I think it's more of an issue with old stuff than with anything new.
Bottom line - you should be just fine with 2v output. The difference isn't major enough (in my opinion) to be a deciding factor when purchasing a head unit. The other things you mentioned, like built in crossovers and Ipod control would be much more important (again, in my opinion), along with a user interface you like, overall looks, reliability, sound quality, type/number of outputs, cool features, etc.
That said, I don't have a lot of recommendations. I love my double-din Alpine, but it doesn't have the built-in crossovers and eq you're looking for.