My guess is you are hearing the sound of the polypropylene plastic bucket that houses the midrange.
Polypropylene cones always had a chesty sound - as if the vocalist had a bit of a cold. It was apparent in many of those designs of the 80’s - BBC speakers and Mission 770 etc. Very popular back then as you punch the cone out of a sheet of polypropylene very cheaply.
The problem is that polypropylene is non-fibrous and tends to store rather than dissipate acoustic energy so it vibrates characteristically - not a good material either for a speaker cone or cabinet for a speaker cone.
You can observe this effect with any plastic bucket when you start filling it with water - the frequency is lower mid range.
Polypropylene cones always had a chesty sound - as if the vocalist had a bit of a cold. It was apparent in many of those designs of the 80’s - BBC speakers and Mission 770 etc. Very popular back then as you punch the cone out of a sheet of polypropylene very cheaply.
The problem is that polypropylene is non-fibrous and tends to store rather than dissipate acoustic energy so it vibrates characteristically - not a good material either for a speaker cone or cabinet for a speaker cone.
You can observe this effect with any plastic bucket when you start filling it with water - the frequency is lower mid range.