When I have time I'll try to take a "very near-field" measurement of the mains and see if the step looks more 'normal' i.e. minus SBIR.
I agree. It would be interesting to see how step and phase response of your mains is looking when there is no XO either in miniDSP and all settings of their internal DSP is set to default. You should examine phase and step response of 20-20kHz and 30-110Hz sweep when they are in the middle of the room vs when they are now. You should check both mains as they shouldn't be major differences between them.
I'm still suspecting their internal DSP..