LTA microZOTL or Backert Labs Rhumba Extreme 1.3?

Keep using the EAR preamp! You like its flavor. That's all that matters! Changing to the other two preamps is a sideways move. 

From what I’ve read I’d guess the LTA and Backert might sound a little less rich and add more upper-end detail — a little more white wine or Rosé and a little less red wine. But that’s just a guess. Member Terry London reviewed both the LTA and Backert (not sure if it was the Extreme model) I think for Stereo Times and slightly preferred the LTA, but it was pretty close as I recall and doubt you could go wrong with either if they deliver sound qualities and/or improvements you might be looking for. Then again, you’ve obviously got a great preamp so read the reviews and make sure you’re not actually going backwards from where you are. FWIW, and best of luck.

I agree with @jasonbourne52.  The EAR 868 is well regarded.  The preamps you listed are likely a lateral move.  If you want to take it up a notch, consider Audio Research REF 5SE... supposedly ARC pre's pair well with PASS amps. 

I own the Micro ZOTL preamp and the Backert Rhythm 1.3. I do prefer the LTA Pre over the Rhumba but the Rhythm crushes them both. The LTA is very good, but if you've got the coin go for the Rhythm.

I agree with @jasonbourne52.  The EAR 868 is well regarded.  The preamps you listed are likely a lateral move. 

No, they’re absolutely not a lateral move.  They’re a “different” move, but whether that’s better or worse depends on what the OP is looking for sonically.  That’s the absolute key.