There might be material applied to the inside of the mid range cones in order to dampen them.
Hard material cones (ceramic and metallic) have a greater tendency to ring like a bell (=distortion) than fibrous paper cones or fabrics . Paper and fabrics can be more easily embedded with viscous damping fluids/materials or weaved in a manner to reduce this form of distortion.
If you suspect a change in the timbre as "open up quite a bit" suggests then you might want to visually inspect the drivers (take them out to see if anything is detached)
Hard material cones (ceramic and metallic) have a greater tendency to ring like a bell (=distortion) than fibrous paper cones or fabrics . Paper and fabrics can be more easily embedded with viscous damping fluids/materials or weaved in a manner to reduce this form of distortion.
If you suspect a change in the timbre as "open up quite a bit" suggests then you might want to visually inspect the drivers (take them out to see if anything is detached)