This is what Mackie told me when I asked the same thing. Although he was misinformed on my MP-1, it will give you an idea of what else you can try.
Personally I like the second option if I use anything at all.
"Thanks for writing. As I understand, the MP-1 is a Microphone Preamp,
and
the Rocktron Xpression is a Digital EFX Processor. If you are
using the MP-1 as a Mic Pre, then you will want the output to connect to a
Channel Line Input on the Mackie Board. The Xpression can be connected
a couple different ways. If you want to dedicate the Xpression to one
instrument or sound, then you want to connect the Xpression to the
insert point on the channel you want to hear the EFX. This will be using
an Insert "Y" type cable as described in the manual. You will have to
tweak the Mix of Wet vs. Dry by ear and what sounds best. If you want
the Xpression to be used for a combination of instruments or sounds, you
will want to connect Aux Send 2 from the 1202 to the Input of the
Xpression. Then connect the output of the Xpression into a pair of Channel
Line Inputs, or into Stereo Auxiliary Return
#1. This EFX Loop will allow you to create an EFX Mix on the board.
All of the Knobs labeled Aux 2 EFX on each channel will be the amount of
signal of that sound you will send to the Xpression. Typically with
this hookup the EFX box will be set to 100% wet. Once the Xpression
receives the signal via Aux #2, it will process the signal and send a 100%
Wet signal back to the board. If you have enough channels on the board
to connect the Output of the Xpression to Channel Line inputs, then
your Effected Sound will be applied to your mix Via the Gain Knob on the
Channel you connected it to. If you connected the Outputs of the
Xpression into Stereo Auxiliary Return #2 then the corresponding Knob will
apply that sound to your mix. The second scenario I just described will
give you the ability to tweak your Wet vs. Dry mix on the board by
adjusting the appropriate gain knobs."
