Thinking of upgrading to ML ESL 11 or 13a. Thoughts?

Been there exactly.  Started out with ethos and upgraded to montis when the new line came out.  Quite a big improvement for me.  If you can swing the 13a, 
I think the bigger panel would be nice.  I have heard them but not in a good room. A lot wold depend on your room.  The 13a are quite large compared to the montis.  Definitely too big for my room.

Yeah, I also like the smaller cabinet design on the Montis. I guess I'm more curious if the panel is "better". I have read that they made improvements, but outside of the bigger panel creating louder sound, not sure if it will improve the quality and sound in general if that makes sense?

My experience is that a larger panel sounds better, not just louder (more detail, more musical, etc). It doesn’t seem to make sense, but that’s how it works.  The Montis is not that much more visually imposing than the Ethos.

Yes my SO did not even immediately recognize that I had swapped the Montis for the Ethos.  They are bigger in every dimension, but the difference is somewhat subtle.  The new Masterpiece series is noticably larger in the bass cabinet.  And yes the larger panels sound better not necessarily louder.  If you can find Montis, I can easily recommend them.  Personally, I liked them better than Summits.  The DSP seems to be the secret sauce for panel / bass cabinet integration.

Great, thanks for the responses. I may try a Tekton Moab out - apparently its great for HT usage...and if I don't like after the 60 day trial, Ill go montis maybe