Having the H90 function in mono naturally by only connecting Output 1 is always better than having an outboard device sum for you. The algorithms are designed to adjust for optimal performance when appropriate connections are made, and they have no way of detecting an external sum mixer. That said, in most cases, you will be okay summing externally, as in the case when your running a stereo rig but you can only connect in mono for a show or something. I’ve done it. Nonetheless, ask the pro rig builders and 100% of the time they will tell you, connect things in mono if that’s what you want, don’t sum externally.