I'm in the process of comparing various power cords and interconnects for my Pass amplification. For interconnects, the standouts are Synergistic Research Element Tungsten and CRL Silver, and for power cords the CRL Mk II (new ones with Oyaide 004 plugs). These handily bettered the VooDoo, JPS, Audience, and Van Den Hul cables that they were compared against.
My personal experience (from owning the X600.5s for several years) is that one has to be very careful in picking cables for Pass gear because they are warm sounding in the first place, and any hint of additional warmth in the cables will quickly make the resulting too colored. The SR and CRL cables mentioned above are among the most neutral sounding cables with Pass gear.