I've got a Tune Zii3 that's my main axe, and I love it. I'm also not shy about basic set-up and truss rod tweaks. The bass has titanium-reinforcement in the neck, apparently, though I'm not sure how this works. Anyway....I switched strings to DR Longnecks a while back, and changed string height, etc., and messed with the action a while back. I've got it set up now just the way I like the action--it's pretty low, but not super-low... the only problem is, there's a strange "sibilance" playing above the 15th fret or so, on D string and all other lower strings....(Puh-leez, no jokes about "why are you playing up there anyway? )- It's almost like a "chorus" sound....like two notes together, slightly out of tune, and sounds a bit "choked"-but the notes sound, and sustain..... I've checked intonation at the 12th and 24th fret and all seems fine, actually. Any idea what might be going on here? I was thinking maybe resonance of the neck reinforcement? Or is it something else?