I've done all of the above, but still way too quiet. If I plug the Moog straight into my QSC PLX2402 I have the same issue. This tells me the Moog's output is not really +4 dBu but something much less.The input sensitivity is right about +4dBu. Note that when usin the aux in, you will need to set the master volume to your desired volume with the aux source then use the preamp gain and volume controls to add the bass into the mix.
Be sure that Moog device's volume control(s) are up sufficiently high as they will reduce the volume from +4.
One thing to be aware of (general comment), if you are using a balanced TRS output from any balanced device, you will need to unbalance it BEFORE going into the aux in. Since the input is summing, it will try to sum the inverted signal with the non-inverted signal and all that will come through is common mode garbage which will be very low and sound terrible.
Guess it's time to look for a preamp for the Moog.