I would suggest as a very inexpensive option a small mixer, like abehringer 802 this will allow you to monitor the mix at the mixer and solve this issue for you. Here is the little guy now http://pro-audio.musiciansfriend.com/product/Behringer-Xenyx-802?sku=631263


HP Win 11 12 gig ram, Mac mini Sonoma with 16 gig of ram, BiaB/RB 2026, Reaper 7, Harrison Mixbus 11 , Presonus Audiobox USB96