During a sunday service I noticed that some monitors (set on mono auxes) appear louder than others with the volume count being equal on both speakers, one is very loud and the other is very soft. Could this be the cables? There is no thresholds sets and there is no preamp gain option availible either.
Have you tried swapping the cables coming out of the AR unit feeding the two monitors? Then you can see if the low level follows the output channel, or the cable/monitor.
Also check the EQs, compressors etc on each aux and make sure that they are also set pretty much the same. The makeup gain on the compressor could be turned way up or down on one or the other.