New replacement arrived this morning. I warmed the pads for around 30 minutes (it was 14 degrees out and they were cold and stiff).
Verdict: The first pair of HP50's were seriously defective. Anyone who got a pair like I initially described should immediately pack them up and either exchange them or put in a warranty claim. It would be hyperbole to say that they don't sound like the same headphone -- but there are some definite, extreme differences. All of the scoop I mentioned is gone, tone and instrument (especially vocal) timbre are all really well reproduced. No more synthetic timbre, no more hollow sound, no more scoop.
Modest Mouse's "We Were Dead Before The Ship Sank" and...everything is *exactly* where it should be. Midrange on "Missed the Boat" and "We've Got Everything" are crisp and clear, with proper tone and timbre. On Sam Cooke's "You Send Me", Cooke is front and center. On the old pair, his female backup singers seemed audibly five feet in front of him; on this pair, fixed. (Getting the sound right on this test track is really important to me. It's the song my wife and I danced to at our wedding.) Re: The Decemberists. Colin Meloy is back where he should be. On "Sunshine" from the Billy Liar EP, Meloy and Jenny Conlee are both pushed forward - rather than Meloy sounding like he was mumbling just behind her. Tori Amos sounds pitch perfect.
<-- This face.
Man these headphones are sweet. Large scale mid-fi headphone sell off coming this weekend/next week of all the others that I've accumulated. Hunt over.
1. I owe a mea culpa to Roomfeel and Barton, as Roomfeel clearly wasn't the cause of the previous problems. They were just plain broken.
2. /Rant: I know that every manufacturer ships out the occasional bad unit, but I really hope that my unit was a rare exception. I wish that I had the old pair here to A/B them more directly and to get them measured. I just don't $299 wish that I still had the defective pair here. /Rantover.