There are few direct upgrades after the lcd-x. From my experience, there’s the he1000se, lcd-5 or susvara. Everything else is more of a side grade but for me, I’ve just found a great pair to complement my lcd-x, the heddphones v1. They have super fast treble extended, even more detail than the lcd x but only average subbass extension. The lcd-x on the other hand are still very detailed, but have huge slamming sub bass for the wub wubs.
I ended mine with only $900. My second hand heddphone 1 has genuine mind blowing flagship level performance in the treble and upper mids, excellent midrange and decent bass performance. Only headphones which beat it are he1000se, susvara, Stax or abyss, and they only do significantly better in the bass region.