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. 

+1  @lancelock 

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.

You can do the two week trial with the Microzotl. I did and found it was not a good match with my power amp.

I don’t have an apples to apples comparison but I was able to borrow the rhumba extreme 1.3 for a week driving schiit aegir monoblocks. I was really impressed by the increased micro dynamics. It also threw a taller and wider stage than my Pass integrated. On the downside, I could not live with the volume control. Just not enough useful range and I was never able to get past 10 o’clock on the dial. 

I went with MicroZOTL L2 Pre. I am keeping it. Never tried Backert but I am sure it won't disappoint either. I did a quick review on MicroZOTL in another thread.

Cheers.

Tons of gain with the MicoZOTL even on low mode. Could never  get past 10:30 with any amps I had. Sounded real good but real tough to just get the right volume. 
Lot of fun tweaking though with different tube and fuse combos,umbilical cords etc. 

Rogue RP 7 was quite a step up though. 

@benzman - not sure which version of the microzotl pre you had, but mine has - 15db and - 30db gain adjustments. Sweet spot for me is the 15db gain setting. With that I can bring the volume up o 50 out of 99 no problem. I sit about 7 ft away from the speakers and listen at about 55 to 65 db.