ESS AMT1B Bookshelf problems.

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I got a pair old ESS AMT1B Bookshelf speakers.
The previous owner replaced the surround on the woofers and the passive radiators.
In fact replaced the flat pieces of the radiators with corrugated plastic sheets, mainly used in the decor business.
The mids and highs are ok , but the bass sounds boomy. Overall the sound is terrible.
I checked the resonance frequency of the system, and it is ca. 78Hz, the woofer free air fs is 45Hz and if I take out the radiator the woofer in the enclosure measures 41Hz.
I see on the specs sheet the original passive radiator marked to have an fs of 5Hz.
The cone of the woofer is extremely stiff, the system rated 375W music power.
I don't need the 375W, but I would like to have some nice bass, to the sound what the AMT-s produce. I ordered a pair replacement passive radiators.
My questions are:

1. Is this woofer supposed to be like that, or I have to replace it with something else and if yes with what?

2. Is there a way to measure the resonance frequency of a passive radiator?

I guess in some way it would show up on an impedance plot.
Thanks for any help or advise.
Miklos
 
The ESS AMT1B Bookshelf speaker and the specs :
 

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