Maggie MMG versus Ls3/5a

IMO, MMGs provide great value at $600/pr., but they do have limitations in dynamics and deep bass. Then again, I'm not familiar with any $600/pr speakers that don't have limitations. Certainly the LS35a shares those very same issues with the MMG. Presumably, the OP uses bass extenders in part to address those issues, so.....

I took a similar approach and use MMGs with subwoofers. Freed of heavy lifting below 80hz, they sound really, really good (to my ear, anyway) - better than any other speaker system that I've heard at the price point, and by a pretty wide margin (again IMO). There are a lot of good speakers at +/- $2k, but a pair of MMGs with a pair of good $1400 subs is that rare option that will also provide true deep bass. Dynamics are very good, tho ultimate SPLs remain limited. Timbre is excellent and octave to octave balance is satisfying, even if the high end may strike some as a touch dry. I find that imaging is also convincing, provided you like the way planar speakers image. (Not everyone does.)

OTOH, to make such a system sing you need to properly integrate the subs. I cross actively and the appropriate electronics will add to the total system cost. There are a lot of ways to skin a cat in this price range, but I do like the MMG+ sub+active electronics approach. Just one more data point for the OP to consider.

To different speakers its not VS its what you like!!

Maggies/planars are top notch on transients I would say with most any amp and "attack" ie the leading edge of a transient in particular.

Some box speakers are as well, but many are not, and amplifier used is a bigger factor here as well I think.

"Monster" SS amps should also serve you quite well to help get those top notch results for "attack" in particular out of mmgs.

30 years ago (so take this for what it's worth) I wandered into a showroom intending to audition and probably buy a pair of Rogers LS3/5a. I listen primarily to orchestral music and value the sweet reproduction of acoustic instruments --and voices-- above all else.
I listened for a while to the Rogers and concluded that their sound was rather beautiful, maybe more sweet and realistically lovely than any other speaker I had then ever heard.
I had read in the Absolute Sound of these newer design Magneplanar speakers and, not having seen a photo, I had assumed that they would be too big for my room. But there they were, MG-1s, newly released,off to the side and clearly, in their slenderness, not too overwhelming after all. So I asked to hear them.
The result was a conflict that, at that time, may have been unresolvable: Beauty of sound vs. power of presentation. At the very first moment, I could tell that the MG-1s did not have the warmth and realism of the
Rogers. I remember that I instantly commented. "they're not as beautiful as the LS3s." But, I was overwhelmed by the size of the image! I was, for the first time in my life, hearing a fair representation of the physical size of a performing group. I knew immediately that they had slain the Rogers and I bought them.
But I always remembered the Rogers and often wished the Maggies would be sweeter.
I now have speakers (Shahinian Hawks) which I feel give a decent representation of both the beauty and the size of music. But, if I were back in 1980, I would still want both designs. You fairly describe the LS3 sound as 'lush.' I believe you will never achieve that kind of sound from Magneplanars
of any design no matter what you do. But, the little LS3s really haven't much of a chance against all-out rock and roll.
From my experience, take it as you will, if you can live with the (fairly severe) volume limits of the smaller speaker, unless you keep them along with the MMGs you will miss that wonderful wash of sweetness in the years to come.
Maybe the thing is to use the LS3s for the smaller stuff and sacrifice the realism for the greater impact of the planars.
good luck.