Switch to silver for a treble boost?

Carl you might want just to try it; for a hundred bucks it's an inexpensive experiment. However in my own experience, switching to silver based interconnect cables (the old Audioquest Lapis; from CDP to pre, and from pre to PA) yielded some extra zip indeed. Cables are always system dependent so you never know without trying them.
Also you may want to experiment with upgrade AC cords or even line conditioning devices, both of which can change sonic signature significantly. My hunch is to start at the interconnects; that's where I noticed the biggest impact on my own rig.

My system isn't resolving enough to warrant an upmarket PC (and I'm a bit of a fence-sitter there too).

I'm quite happy with the sound of my CD player and amp via a pair of Grado h'phones; it's via the speakers that the system seems to lack treble sparkle, and I guess a $100 pair of speaker cables is cheaper than changing the speakers (which I'm otherwise happy with for bass and mids).

This really begs the question, what speakers are you using?