I own all ARC gear, a Ref 5se, Ref Phono 2se, Ref CD7 and Ref 110 amp. I have tried a number of different interconnect and speaker cables as well as power cords. My favorite are the Shunyata Zitron Alpha HC. Sane cables might be slightly better. I did not have them at the same time as the Shunyata. They are very similar sounding. Both the Nordost and kubala-sosna cables I tried seemed to change the sound in different ways. The Shunyata is more like opening a window all the way. It does not changed the way my Ref 5se sounds but enhances it. One of the cables I did not try was Wireworld. I have heard good things about their cables with ARC gear.
The Shunyata Zitron Alpha I have on my Ref 5se is a huge step up from the stock power cord. You will not truly hear what the Ref 5se / Ref 6 preamps can do with the stock power cord.