I've heard it on a lot of albums. It's not so much an echo as it is a "scoop" - I'd best describe it as a produced hollowness in it's midrange reproduction. I'd put money that this is a consequence of ROOMFEEL, by attempting to voice the headphones to sound like a pair of speakers in an acoustically treated room (taking Chamber properties into account) the changes have the effect of shifting some vocal production to produce this sense of space.
From my review notes, because I won't have my HP50 back until tomorrow at the earliest by UPS:
If you get out The Decemberists album EP "5 Songs," you'll notice that Colin Meloy's unusually forward nasal vocals are now set back at a distance. (By the way, this had a pleasant effect on this album. "Angel Won't You Call Me" has never sounded better.)
This is a pretty rare exception of it having a pleasant effect: it's very troublesome on other tracks/albums. On the Sam Cooke collection "Portrait of a Legend 1951-1964," if you pull up "Having A Party," Cooke's vocals are buried even more behind the baseline than the HP50's frequency response would suggest. He sounds at a far greater distance...and there is something unnatural in his vocal timbre. Similar timbre problems pop up with Tori Amos; her album "Little Earthquakes" sounds pretty off through the HP50's. Best example track, "Winter."