I like the phrasing of your question, i.e., "...natural sounding...". Of the several ICs I've listened to over the years (including Cardas, Jena, Silversmith, Valhalla, HMS, TG, Stealth Indra, etc), my favorite is the Cerious Technologies - by quite a large margin. Good luck in your quest.
ICs with great natural sounding highs?
I currently use Acoustic Zen Silver Reference II (XLR) and it's a great IC but the highs have a certain "silver" coloration especially in the cymbals sound. It's not harsh or bright but when I auditioned Cardas Golden Reference ICs recently I was struck with how cleaner and more natural the cymbals sounded - you can hear the real metal when the cymbal is hit rather than a splash of silvery noise. In all other aspects (bass, sounstage, details etc) however Acoustic Zen came out as a clear winner. But I would love to get a cable with absolutely realistic sounding highs as well as great soundstage, neutral mids, detail and bass that Silver Reference ICs do so well under $1000 for XLR. Should I try Synergistic Research? Shynyata? Others?