Used cables $1k - SF Amati and Bel Canto RF1000??

I have good luck with my Synergistic Teslas (XLR) between my Pass XP 10 and my Ref1000Mk2. Cardas Golden Ref work well too for me. I think the Teslas give a little more oomph and open the sound up a bit more.

That would be Purist Audio, of course, though $1k might not quite cut it. Some would say Jade Audio too, I guess.

Just want to say that when it comes to balanced IC's be sure to take it real slow. There's a lot more disparity in performance among single-ended IC's than balanced (exactly why I've never found a satisfactory answer). My point is you may be able to find extremely suitable balanced IC's at big savings...the loaner cables available from most of the used cable websites may be worth their weight in gold here.

Agreed. The fact that you need balanced cables both simplifies and complicates the matter. But speaker cables are going to cost you in any case. However, why don't you try first DiMarzio Super M-Path speaker cables? They are close in character to Purist and would cost you about $600 new. Read couple of threads about them here. I personally don't use them, I use Purist Colossus, but if I were in your situation I would certainly try them and some used Purist.

I hate to sound negative, but I can't think of a more foolish way to go about fixing your problem. Trying to fix a problem with cables is like buying an EQ with one setting and you don't know what the setting is until you actually try it. I would suggest going a different route to fix this problem. I know this sounds crazy, and that most audiophiles wouldn't even consider doing something this foolish, and would even fight me every step of the way; but if it was my system, I would find out what is causing the brightness in the system and fix that, as opposed to buying random cables.