Need help Choosing A New Interconnect

I have a McIntosh C2500 Tube preamp with a McIntosh MC302 solid state amp driving a pair of Golden Ear Triton References speakers. I have had this system for a couple of years and I have noticed that that sparkle in the highs is gone. I noticed it after I replaced my half as powerful MC152 for the MC302. I do know that the MC302 was a little soft in the highs as read in reviews and talking to other McIntosh owners. I thought it may be tubes, so I rolled in a few different brands, some NOS and some not in the tube preamp. Although I heard differences, it wasn’t what I was after. I bought 2 new McIntosh branded tubes for the line stage and they broke in fine but it sounds the same as it was originally. I have tone controls but prefer not to have to use them, but if I do I have to put the treble at +2db..

I tried a couple of Nordost cables but found they made the entire system sound lean. I use a full loom of Groneberg Quattro Reference cables and really like the sound. I want to get a cable that won’t alter the bass or midrange and just give me that little bit of sparkle in the treble that is missing. I have already tried Nordost Red Dawn interconnects, Red Dawn Leif series interconnects, Audio Art IC-3 interconnect, Audioquest King Cobra Interconnects, MIT AVT1 interconnects. I also tried a Kimber Silver Streak but it made the entire system too bright.  None of these cables have given me what I want. Also, whatever cable I get needs to be 1.5 meters with RCA's all ends and sell for not more than $500.00 new. Used would be even better as I could get a lot more for $500.00. I am looking for suggestions from you ladies and gents from what I have stated above. I would appreciate some help...………..Thank you.