I've now confirmed that it was simply a memory bias for me. I've had 3 pairs of Sines, not all at the same time, and I thought my first one was the best. I sold it because I thought I was going to get another Mobius pair but ended up missing it. Since then I also listened to a pair of EL-8 Closed that I really liked. A couple months without my Sine, combined with listening to other headphones, made me remember the original Sine incorrectly. So when I got my second pair, I thought it sounded worse--not as good as my original that I consider to sound nearly perfect to my liking. When the recent refurb/open box sale happened at Audeze, I grabbed another Sine pair, to keep as a spare or to gift. I expected to hear some difference between my second pair and this new Sine, but no, they really do sound identical to my ears. I've also asked Audeze about this, and they said there were no major revisions with the Sine, and all pairs should sound similar.
Memory and other psychosomatic effects on our impressions of sound play a big role in how we perceive our audio equipment. There's even a known effect of listening more closely to new equipment, which makes it seem like the sound is more "detailed", but it's actually us focusing on any perceived changes and minute detail.
P.S.: if you want a more "sparkling sound", use the Cipher cable with an iOS device (or regular audio cable + amp + iOS device). The DAC and DSP in the Cipher cable do process the sound in a certain way to give a greater apparent separation and detail and a brighter, more sparkly signature. That's in comparison with a few Android phones I've tried and a separate DAP. I have a couple of good speaker set ups at home and realize how much brighter the Cipher sound signature is in comparison to a more powerful and full, detailed sound of my speaker set ups, so I sometimes swap to iPhone + Cayin C5 amp + Sine instead of using Sine + compact Cipher cable + iPhone. And yes, I literally got an older iPhone on eBay to serve as a DAP (and now as phone) because I liked the sound that much more--I was using Android up to that.