Psyre wrote:I'm mainly looking for a little additional headroom as I'm not able to get quite as much presence as I would like.
How are you going to go about summing the external preamps with the native Mackie channels?
Take this with a grain of salt, as my first hand experience with Mackie desks is fleeting (I've used them but don't own one), but unless you are hammering the shit out of it, I think the point at which the Mackie runs out of headroom is post channel strip - in the buss or master sum sections, so.. if you're hanging externals off a direct in to the Mackie you might find yourself still fighting this particular goblin.
Honestly, I'd try and borrow something and confirm it improves things before sinking too much money into it.