I used ML for years both Aerius and then Ascent/Decent.
The panels and the current state they are in are an issue. When new they can sound pretty awful and they really do need a lot of time sometimes to break in. Hence why you had trouble with 13A I suspect.
I went through a lot of panels. No pair lasted more than 7 years. I used them for 17 years in total.
What's really odd is some replacement panels actually sounded great from the word go, and some would take ages to break in.
It's a really fine balancing act but when they are running right i.e. the membrane tension is actually right they can sound fabulous.
I thought Ascent was better than SL3 TBH but hey if you like them, who cares? Why? Better bass/panel integration. But that's always an issue with any ML hybrid. They can all sound a bit thin because the bass emanates from the bottom of the speaker with a huge panel only doing mid/treble. What I mean by that is there's no bass coming out higher up the speaker to balance the sound out. Maggies and Apogees never had that issue.
I also thought Summit really wasn't any better than Ascent/Decent.
They can sound excellent with a valve pre and a powerful valve or solid state amp. Certainly if the SL3 really are on form they will be outstanding value for money these days.