I'm going to post my results from my experiments to solve this issue, that way it could maybe be useful for someone else who finds themselves in the same predicament.
I've thoroughly tested all equipment, bringing in alternate pieces when necessary (speakers, integrated amp and cd player in place of media server) and the issue doesn't change, so it's not the usual suspects (amp, source or speaker). I shut off the breakers to the rest of the house and verified it's not an appliance on another line. I've tried a variety of recordings to make sure it's not just an issue with one recording.
Following the advice of Grant from Shunyata I uncoiled my speaker cables and ran them straighter, without any coils to the speakers and let them settle back down. This was the first thing to have positive sonic results in my experiment. Sibilance isn't gone altogether, but treble sounds more open with zero distortion. Playing Marie Laveau from Kenny Barron's "Things Unseen" Album, the trumpet sounds still a little piercing, with sibilance on highly extended passages, but better than it before. Then, I took out my Nordost IC and took it over to my dad's to test it out in his system (this is the newest cable I added) thinking maybe something was up with it, but it sounds frigging amazing. I bring it back home and do an A/B with the purist IC and sure enough, the sibilance is only noticeable on recordings like the kenny barron that less than stellar trumpet production work (IMO). So I think it comes down to the Nordost being a bad match for the Dali's. When I first put it in, I was really in love with it, and I'm feeling a bit foolish for being so quick to jump up and down for joy. I think the Dali's are just too forward for the Nordost, which is a bummer because I love this cable. In any case, I still plan on doing a connector cleaning and further isolation tests to make sure, but I'm heaving a pretty big sigh of relief.
Thanks everyone for your help.
