There's a fundamental thing that needs to be understood here....the difference between microphone level and line level.
The signal coming from a microphone is incredibly low--typically around -50dBu. If this was expressed in voltage terms, it's something like 1/20,000th the level used inside a mixer or whatever.
The job of the pre amp is the amplify this signal up to "line level" which is the level used inside mixers, outboard rack gear, CD players, etc. etc.
When you plug a microphone into your mixer, the very first thing it hits is the mixer's version of the pre-amp and the signal is raised to line level. The inserts come after the pre-amp because, when you feed the signal out of the mixer to whatever outboard you want, it needs to be at line level--and the return is at line level too.
So, forget feeding several auxes to a pre-amp--the auxes are after the mixer pre-amp so you'd hugely overload a mic pre if you fed a line level aux into it.
Forget feeding an insert out to a mic pre-amp. Again, it's after the mixer pre and would overload badly.
So...if you want external pre-amps, you have to have one per mic and feed into the line level sockets on the mixer--or, even better, as somebody suggested, drop the mixer entirely and just feed the pre-amp to your interface.
However, if you're basically happy with your sound, why worry about an external pre-amp at all? As I posted yesterday, your microphone and your room acoustics make a far bigger difference than a boutique pre-amp. Unless you have an excellent microphone and spend a LOT of money on the pre-amp, you're just wasting your cash.
Edited to Add: I just noticed the original date and the update just above...I was responding to the first page of posts. Ah well...maybe this is useful to somebody searching threads!
Bob