uneven sound

have noticed lately that my speakers have sounded a bit dull ..not as bright in one speaker..i have swapped channels , changed speaker wire, changed rca,s and have determined that the problem is the mid bass driver on one of the speakers (the mid driver handling most of the vocal work) ity sounds flat compared to the other ...but i cant tell if its the driver of could it be the crossover?
How old are the speakers?
If they are older, it could be a capacitor losing it's value. A quick measure with a proper meter can check that.
If it's the driver it's likely worn out. Either from age or it got a hard hit that it didn't like.
As the moving parts of the speaker are used they become less resilient, at which point they have issues reproducing the higher frequencies in their band range.
Depending on the design of the driver and box, a simple light touch/push on the cones can show a difference in physical resistance under your fingers. If you feel the flat sounding driver is a bit loser under your fingers, a recone is in order. At which point do drivers on both sides so they sound exactly the same again.