I have this set up and the way I have it done – just the outputs 1,2 of the h90 into the line in of the stomp – does mean that the amp/cabs I’ve tried have resulted in a mono signal. Probably because the IRs are mono.
The HX Stomp’s signal path is stereo, however, if you feed that signal into a mono block, then both channels of audio are combined and sent out of the block as mono. IRs don’t decide if your signal is mono or stereo, but the amp/cab you choose does. Make sure you use a stereo version to preserve the imaging.
I assume the same issue is present in theory when using an insert but suppose if you’re inserting the stomp before the h90, this is a mute point.
That is correct.