That is, indeed, one thing I evaluate with that track … but I use it even more for assessing transient response (plucked strings), attack and decay, as well as tone, timbre and micro-dynamics.
It’s hard to say what you’re hearing. I doubt it’s down to bass distortion in that particular piece. It might be that you’re hearing the natural micro-dynamic reverberations in the strings, which the HD6XX are more capable of resolving than the LCD-2. In such a case it would be audible as more texture in the notes that might be smoothed over by a less resolving transducer.
For reference, I owned the LCD-2.2c for about 5 years, along side the HD650 (and many others) and as much as I loved the Audeze can, and as many things as it does well, micro-dynamics were never it’s strongest suit. They have a tendency to smooth over them somewhat making them harder to discern/dulling their effect. And, to be fair, I find that to be broadly the case for planar cans in general … typically they’re behind similar dynamic cans in that area (but then again, they tend to do a better job with bass presence and slam).
One way to see if that’s the case is to compare against other dynamic headphones that have excellent micro-dynamic resolution and see if the grain is also audible there. Otherwise I’d have to listen with the same chain to have a better idea.