Well, what do I know? I wasn't getting a good seal!
After wrestling with the headband for close to an hour, I finally got a perfect seal with the W1000X. And wow, what a difference!
Midrange became thicker, bass became weightier, and treble is no longer intrusive. So I switched the L3000 pads back in, and I'm happy now. There's just a little bit of bass resonance that makes bass less distinct compared to the ES10, but I guess I can do Purrin's mod at any random time to address that.
I can hear a difference between the Leckerton UHA-6S and my various sources again. I thought the amp wasn't up to snuff for the W1000X, but I was sorely mistaken.
Get a thick, short rubber band and put it around the wings, perfect seal every session.