Moxified, I had a damn strange problem with my HTDs... It sounded like a bad midrange driver, kind of sizzling during playback of certain sounds. High-pitched string instruments would always do it... bells, chimes, the high ranges of speech, that kind of thing.
I went through two driver replacements, which HTD assured me was damn strange. Drivers rarely fail. They were also emphatic that it would not be the crossover.
In the end, the culprit turned out to be my Pioneer AVR. When I switched to an Onkyo, all my problems went away, and stayed gone.
There is a lot about my case that didn't make sense. Neither my nor HTD really understood what was going on--best guess was the Pioneer had too much DC offset for the HTDs to handle. I even exchanged Pioneers, and the replacement had the same issue... But the Onkyo did not.
So, based on my experience, I suggest you find a way to drive your HTDs with a receiver of a different make, and see if the problem manifests any differently. Yeah, it is a long shot... the sound you describe isn't like the distortion I had. But it's something.