Aside from the breakin possibility that has been mentioned, the one other possibility that occurs to me after looking at descriptions of all of the equipment you mentioned relates to the characteristics of the SR speaker cables. None of their basic technical parameters appear to be specified (e.g., inductance, capacitance, resistance). Given the impedance characteristics of the speakers, which are fairly typical of ML hybrid electrostatics and are shown for the Summit X in the graph near the bottom of this page
, if inductance and resistance are not low it could easily account for the symptoms you described. And the longer the cables are the more likely that is to be true, as inductance and resistance are both proportional to length. Capacitance, which is also proportional to length, is much less likely to be an issue unless it is unusually high.
You might want to contact SR and see if they can tell you what the values of those parameters are, either per foot or for the specific length of your cables.
Good luck. Regards,