First of all, great review, like always.
For the biggest part iFi’s xSpace, or 3D+ on other devices, is a crossfeed. Sometimes with some little extra treble energy, yes. So if you wanna know if the Go Bar xSpace function is doing anything I would recommend measuring the Left channel while only the right channel is playing, with xSpace on and off.
I did something similiar some time ago with my xCan. I measured directly from the amp output into a line-in. Not sure if you can do the same but of course you could just test with an IEM and your coupler.