Hey @ericrosenfield,
How is the new Elite Sheepskin pad treating you? We should probably discuss everything related to the HD660S in this thread.
As an exercise – ergh, I knew I was going to regret – I tried once again the Choice Leather pad with the HD660S. It took me 2 songs to realize sound was weird, probably because the first song was not a good sample.
It was very easy to spot the muffled sound with the track below. Go to around 3’15" and wait for the guitars to kick in:
Went back to the Earpadz Velour. It’s still a little bit muffled, but forgivable to my brain. The “best” pad I have here is probably the one I have in the 58X. My ears literally touch the inner foam and I feel no pain anymore because of the callus cultivated in both ears. 
Cheers. 