I have heard the Sierras before (been a while though, back in 2007) as well as the NX2's, so I'll at least give this a shot. From what I remember, the Sierras were a very nice speaker, pretty balanced and solid overall. Don't recall them being bright or anything that would worsen someone's tinnitus, so I'm at somewhat of a loss for what happened in your case. As for the N2X's, they are traditional Danny Richie speakers. I have heard them and liked them a lot, and I own a pair of O-3's, which share a lot in common w/ the N2X's. I found both to be very easy to listen to and rich in detail. Neutral and natural are the terms that come to mind. So, I don't think that the N2X's would worsen your tinnitus, though as I said before, I don't know why the Sierras would either.
Meanwhile, I could see the Dali Mentor Menuets I use nearfield in my computer set up worsening a person's tinnitus. It's not that they are bright, but they also don't back down on the crispness of the highs and are strong in the area of microdynamics. Thankfully, the Dalis don't bother the mild tinnitus I have.
I hope this helps.