Celestion SL6S

I have owned two pair of the SL6s, and I do believe that the treble is slightly rolled off, but not nearly as much as the copper domed original SL6. Still, it's a smooth non-metallic treble to me. Have you tried experimenting with cables?

no clue but I still have my Celestion 5s and they sound as good as ever

on a related note , i didn’t know that Celestion UK made home audio stuff when I came a cross a small 8 or 10 inch subwoofer in Best Buy on a blowout price for less than $100 USD ... wow that was an awesome sub... only 100 watts but I really enjoyed it.

My tale involves my SL600's, not the SL6. I found them to be a bit bright, even with my Quicksilver Mono Amps. String tone was always just a tad thin. I also somehow managed to kill two tweeters.  Luckily, my dealer was able to install replacements each time. As a side note, one of the speakers fell to the floor, one day. Did I drop it? Or did a Southern California earthquake knock it off its stand?  The fall bent one of the speakers' corners, but the thing still worked fine.

Yeah, one more installment in my ongoing saga of stereo woe.

I haven’t tried other cables; however, I would say my cables are fairly neutral.  My USB cable from the MacBook to the DAC is an audioquest coffee.  The interconnect, power cables, and speaker cables are DIY Helix Image.  I would say they are extremely neutral with great imaging and texture that allows for great soundstage separation.

Also, other speakers don’t roll off like these in the same system.  Could some of the old crossover parts be shot?  Could the tweeters be tired even though they are playing?