Switch to silver for a treble boost?

There is little doubt that as a rule thick copper cable will tilt the music energy to the bass and mids overwhelming the treble. I am uncertain about what surgical silver "core" wire is exactly. My first experiment would be thin gauge copper Zip cord. The thinner copper cables are IMHO much better balanced than those whales intended for subwoofers.
If you are committed to silver then I would look into these.
Try the Signal Cable silvers or the Home Grown which comes in various thicknesses and prices. I must tell you that a real silver cable of that length will cost a lot more than $100 even used. You can generally find a lot of bargains for used cable here. It seems that trying and not liking, then selling a cable is a common practice. The fact that it wasn't for one person's taste is irrelevant you may love it.

If you really want to change the stereo's sound then change the Stereo. The whole kit and kiboodle.
Use an old all tube pre with out of controll gain like my ARC (idle for now) Then employ a class D switching amp like the 500 wpc Spectron III its got digitistis in a minor key only. Then Spend a few bucks on using Silver lace cables from Home Grown audio. They are not really in the same league as Signal and I own both.
Finally make the funeral arrangements for the Maggies get a spine and just let it go. Then buy some enormous German horns that sound like a razor. No blankets just nice clean Avant Garde Trios the German horns. Hide the Maggies elswhere for a year or two or else you go back to Momma.