Martin Logan Quest Z Bass - can it be fixed ?

Adding a sub makes it worse. It's a phase coherence issue.
The "speed" of the panel and that of the cheap cone are vastly different.
Tried it before... biamp, muscle amp, cables, you name it.
They play certain things right but once it gets to bass integration they fail miserably. Use a muscle tube amp to drive them and you lose the finesse of the esl panel.
Of course back in those days I didnt know how to mod crossovers.
This crossover would involve a lot more than cap swaps.
Anyway need to know what sonic signature you are after and what amps you have. You might be better off getting a Thiel 3.5 for cheap and replacing a cap or 2 in the crossover.

I've heard the Quests sound good and very bad in different set ups , be careful what you hook up to them . The bass was not the biggest problem , they can sound extremely sterile , even with tubes . A Pass Labs XA160 amp seemed to be the best match .
Johnsonwu 's advice seems sound .

When I get them, I'd drive them with an Onkyo M-504 I think. Maybe I'd agree somewhat with 'sterile', but I really like everything but the bass on them. Crystal clear but no 'ultra-highs' that make it fatiguing, like some other speakers I have.

Are you saying, Johnsonwu, that the bass is fixable with x=over mods ?

Can the panels handle bass (at all) even at low volumes ?
I ask this as I'm wondering if a DIY'er might send everything to the panels, and just watch volume levels.

One more question, did ML fix this with the Request ?
Or, what did ML make that sounds as good as these (but bass) that is in the price range ?

Thx again...

you must protect the panels from bass energy.
Drive full range = disaster.
YOu may disable the woofer and custom build a woofer for it.
The SL3 is the most successful in mating a relatively quick woofer for the panels.
Not so with Quests and Monoliths.
Crossovers can be fixed with better resistors and better inductors to give it more speed and make it less lossy.
Very very involved work with lots of listening required.