A lot of moving parts here. IMO re-paneling and burning in are both oversold here, except burning in not so much with ESL panels. It's possible the diaphragm tension isn't quite right. Burning in is going to lower the tension, lower the resonant frequencies and probably, cause the bass to be even more overwhelming. The room you are listening in also has a huge effect. Subjectively, the increased bass may account for the impression of less sparkle. For what you pay ML for replacement panels, I might expect you get tighter control of the tension than with a third party refurbisher.
Comparing one model ML at a hi fi shop with another in you home has, as I say, too many moving parts to draw any conclusions.
Also, as others have suggested, changing to active crossover could give you much better control of woofer/panel blending and frequency response.
I couldn't agree more, Leporello. Your comments are thoughtful and on point, in my opinion.
I would like to expand on your points a bit, if I may. Although my experience is mostly building wire-stator ESLs, I recently refurbished a Martin Logan panel, which was a learning experience for me.
Assuming adequate diaphragm conductance, a panel's treble response and tonal quality are predominantly determined by the step-up transformer and diaphragm thickness. Transformers with high winding ratios (>100:1) typically give poorer treble response. Hybrid ESLs with closer d/s (diaphragm-to-stator gap) can use lower ratio transformers (typically 75:1), which give better treble response.
Assuming perfect transformer response, a 12-micron diaphragm will start rolling off the treble significantly lower than a 6-micron diaphragm. In any case, you're just not going to get flat response to 20kHz with a 12-micron diaphragm.
No doubt, ML opts for a 12 micron diaphragm because it's robust, they tension it quite high, and a thinner diaphragm could easily tear on installation, which would slow the manufacturing flow.
Most people can't hear 20hHz anyway, so if the diaphragm rolls off the highs at 12k, very few would notice. Still, you do get a bit more treble accuracy and sparkle with a thinner diaphragm.
I would doubt any refurbisher who claims better than factory response, but if everything else is done right, a thinner replacement diaphragm could be a valid basis for such a claim.
Also; ML has a specific, repeatable process for tensioning the diaphragm, which they don't share, and it would be difficult for an inexperienced refurbisher to exactly reproduce the factory tension.
The diaphragm tension is a big deal because it determines the diaphragm's drum-head resonance, and ML will have tailored the crossover filters and spacing between spars to accommodate that specific resonance frequency. If a refurbisher is off on the tension, the resonance won't sync with the crossover or spar spacing, and won't sound as ML designed it.
That said, a refurbished panel with a correctly tensioned diaphragm, should sound wonderful.
I particularly agree with Leporello's observation that a bass-heavy response or tuning, will make any panel sound comparatively muffled and lifeless.
Replacing the dividers (ML calls them spars) should not affect the sound in any way, but it might create some problems if replacement spars are not correctly bonded, such that gaps exist between the spars and stators, or the front stator spars and diaphragm.
The only reason you might want to replace those spars is if they were damaged or dis-bonded during panel disassembly. BTW, those spars are clear, hard, non-elastic polycarbonate plastic bonded to the stators and diaphragm with a 5-mil double-sided adhesive tape.
Finally; I would never use a passive crossover in a hybrid ESL. A passive crossover is at best a power-robbing, phase-altering, non-tunable compromise to allow the speaker to be driven with a single amp.
Manufacturers use passives because it makes business sense, as the lion's share of buyers want an inexpensive plug-n-play speaker that doesn't require bi-amping.
If I owned a ML speaker with a passive crossover, I would bypass the crossover entirely and bi-amp it with a DSP crossover. Not only will the bass tighten-up considerably with the amp directly coupled to the woofer, but phasing errors disappear, the full sonic spectrum sounds more alive with greater punch, and the tuning capabilities are infinite. BTW; it's easier to do than most people imagine.
My 2 cents...
