If I go Hammy I'll bring mine and we can compare.

I usually only post when I have something interesting or constructive to add.  Which doesn't happen much cause most folks here are more knowledgeable than I.  The recent talk of the HD600/HD650 and the comparison to the LCD2 or LCD3 got me to de-lurk.

I want to say it again but I think a lot of this 'plankton' that everybody keeps talking about with the Sennheisers is a result of their smooth response and healthy amount of treble energy around 4-6khz.  The treble in this region helps greatly with ambient cues and room reflections, so the Sennheisers do a good job at 'painting the room,' if you're looking for those kinds of cues in your recordings.  The Audezes by comparison are either a bit recessed in that region (pre-fazor), or all over the graph in terms of sound (fazor).

I'm not sure so much about the frequency response, given to my ears the HD600 is a bit elevated in that 2-6KHz region where as the HD650 is not. Unless you were just going for, "both have a greater presence here than the Audezes." Which they probably do. Granted, the generally balanced, smooth FR of both certainly don't hurt and probably do play at least some sort of role in plankton retrieval. I think it more likely comes down to low distortion, quick decay/clean CSDs, and so on when coupled with the FR, though none of those separately or together necessarily paint the full picture either.

I own a HD580 and HD600.  Both from about 15 years ago.  I also have an LCD-2r2 from Sept 2011 (it's all wood, no metal triangle cable mount thing).  My LCD-2r2 is probably on the veiled side compared to modern LCD-2.  I haven't done a lot of comparisons with other LCD-2 versions against my LCD-2.  I don't have access to enough other LCD-2's to really be doing that. Even with my potentially (and likely) veiled LCD-2r2 I hear more micro-detail with it than my HD600 or HD580...

So, given these are older HD580s and HD600s, maybe the newer versions of the Senns would sound less veiled to you?

HD650 is a tad bit bassier, tad bit darker, potentially slightly better refined, but it doesn't have the slight emphasis in the 2-6KHz range like the HD600 that may or may not bother you (doesn't bother most).