Bill,
Are you already using the MON(itor) output?
If not, I would use that output because it is possible to use the sub for the channels you like. E.g. the vocals do not need sublow...and will only raise the chance to get feedback if you would.

In my setup only the Tyros sends to the sublow-speaker through the AUX1 output of the Mackie 1402VLZ.