Switch to silver for a treble boost?

Alun, after a few weeks of the silver speaker cables (locally hand made from "German surgical grade silver" according to maker), I have to agree. The difference from my previous heavy copper was small indeed, but I accept that could also be system related.
The silver has produced a slight lift in the upper-midrange, which has helped nicely with vocal clarity, but certainly didn't produce any extra treble distinction or "sparkle" that I was hoping for.

I would add that these cables (2.5 meter pair) were only $100, but given my system I'd be reluctant to spend more than that.

Try cables with lower capacitance. There will be less HF rolloff

Carl109, you might also try something with a Teflon dielectric (e.g. Kimber 4TC or 8TC) or no dielectric at all like Anti-Cables.

The way you describe the effect that silver wires had in your system is perhaps consistent with my experience in hindsight. I was incorrectly attributing increased focus and resolution in the higher registers with adding sizzle and "sparkle". For me, more resolution is good, while more "sparkle" is only good if is also balanced with the rest of the spectrum, accurate, and doesn't come at the expense of added sibilance.

Ultimately, a change in gear to equipment that favors the upper registers may be in order for you. Focal, Totem or some other speakers with a bright finish may be the ticket. The English love their mid range, and B&W 600 speaker line, while not as mid range heavy as some, are not as hot up top as some others either.

You mention the 'blanket over speakers' sound. Perhaps it is not due to the cables but to their placement. Tried maple stands? I have padding/carpet over concrete slab and used 2 1/2" maple w/ brass carpet spikes for my spkrs and what a difference. I purchased a workbench from Sams and used the maple work surface to homemake 15x12 platforms for my floorstanders. Have plenty of the wood left for amp stands. Puchased some brass spikes from PartsExprs and t-nuts...bargain upgrade. Not huge...but audible for the better on music/video.

I have short steel stands (short to keep the tweeter at ear-height when seated), which have four brass spikes. Each stand then sits on a cement paver, which in turn is on my timber floor.
The cement paver was an attempt to isolate the speaker from my wooden floorboards and reduce the floor resonating with the bass, which worked.
I think the treble issue is more to do with my compromised hearing sensitivity at high frequencies; if only someone had told me to wear ear-plugs when I was younger and playing in rock bands! Oh well...