I believe the “channel source” setting for an aux is universal and not changeable per channel. That being said, a very helpful work around is to double patch the lead vocal into a second input channel. This way you can EQ the second channel just for IEM and the first channel for FOH. Or triple patch it for different EQ for FOH, IEM, and the artist’s IEM, etc. Of course this assumes you have the spare input(s) to double patch.