Sub Issues with Pulse Soundbar Plus and 3rd Party Sub

Hi, I've been using my pulse soundbar with a REL sub for a while now without issue. Tonight however I thought it was getting a bit late to have the sub on so turned the sub off, before I could disable the subwoofer option in my home theatre network I noticed the soundbar was sounding quite hollow without the sub. When I turned the sub setting off I noticed the soundbars own little sub started to kick in and it sounded much fuller. So my issue is thus, is there a way to have both my REL and the soundbar sub on at the same time? I don't seem to be able to access any sort of cross-fade option for the soundbar through the blueOS app. Thanks for the help!