I have owned the X-250 and X-1. What is descriped above is accurate on their sound. When I tried the combo in a buddies system, it did not work and I heard the brightness that you heard. If I recall correctly, it seemed to come from the matching of the Pass gear to his speakers. There seemed to be a certain frequency that was moved forward in the presentation that made the system sound bright. I was using an Elrod EPS Sig 3 power cord so I felt that it was not the power cord. It also could have been the speaker cable and ICs. They were AZ Sil Ref ICs and Home Depot speaker cable. The Cardas Golden Hexlink 5-Cs seemed to reduce the edge or brightness. I'd try a few different cables to see if you can get ride of the brightness and if they don't work, it could be a speaker matching issue.
Happy Listening.
Happy Listening.