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.


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https://www.ebay.com/itm/123960121461

However the plugs on that one are not the best ones, IC-300 is great with Eichmann Bullet plugs, I'm really not sure about LAT original ones...I have two such. They give that sparkle in the highs without sacrificing bass presence or extension. 
Sorry if this may sound unhelpful but using the interconnects as a tone control is really no different from using the tone control(s) on the preamp. Actually, the controls on the preamp (probably) provides more "control" over the tone than any interconnect can provide.