Why not just EQ in Garageband? If you EQ on the mixer, then that EQ will be permanent. Better to record the signal with as little changes as possible so you can always go back and change the EQ later if you need to. EQing on a mixer is mainly for analog recording. Also less circuitry means a better signal to noise ratio.
First off are the incerts before or after the EQ stage? If they are before then it's all moot.
There is some advantage to doing the EQ before the A/D stage. Yes it is permanent but you reduce noise if you can bring the level up to the point where it uses the full dynamic range of the D-->A converter. I think you want to compromise and think about what is important
I think setting the EQ to zeroes does not reduce the about of circuitry. Unless you bypass the EQ you are still running through a few extra op amps.
That said, I bet the best sound will be had using the preamps on the m-audio interface unless you have a really nice mixer
About my comment to use the mixer's L and R outputs....
OK you have four mics going to the Utra's XLR jacks. Put two more mics in the mixer and pan one hard left and the other hard right. then send the mixer's stereo left to the Ultra's channel 5 and the mixer's right to Ultra's #6.
Using the stero out rather then the incerts lets you for sure use the mixer's EQ