The thing is Mule Driver that normally digital and analog meters are different. So if you send 0dbfs (digital) to your mixer from Logic it's not 0dbvu (analog) on the mixer.
dBVU is what an analog mixer uses for reference, 0dBVU would be where the mixer works at it’s optimum. Noise is the least, while having enough output to drive the sound card.
dBFS is what a digital sound card uses for reference. 0dBFS is where the sound card will reach it’s maximum and if pushed beyond that it will badly distort. There is no “optimum” level in the digital realm per se, as long as you are above 16-bits of usage.
You should check your soundcards manual and also Mackies manual I guess .. I don't know if in this case the Mackie is kaputt, do you get enough signal when plugging in synths etc?