Original MMGs: How know when they need servicing?

[Sensel]

Bought them the year they came out. Work fine. But getting on in years. Heard some people send in for some component updating and servicing.
Cost?

 

[GD70]

If they ain't broke, don't touch them. You start to hear buzzing at certain frequencies, or loss of highs or lows, most likely the coil wires are starting to delaminate or have a break and will need the wires replaced. I have a pair of SMG's that need comete wire replacements, no sound at all, and the fuses are good.
There's plenty of videos on what to look for and how to repair them, or pack them up and ship them to be restored at the factory.