After 2 years of great sound reproduction, my Marshall Monitor’s stereo failed. Not anymore under warranty, www.marshallheadphones.com support would only tell me to get it repaired by a specialized repair shop.
But then I got a brilliant idea!
If I plug it on the L side, I only get sound on the left, if I plug it on the R side, I only get side on the right, but the speakers worked perfectly.
The issue is between both speakers, probably in the hidden cable inside the headphones.
So why not trying to plug both headphones at the same time with a T (male jack/double female jack <-> one jack from each speaker to each female jack) !? And it magically worked!
I hope it’ll help you!