Pros: Superb in every aspect except...
Cons: ...ever so slightly bass extension
I'm a rabid audiophile since the early '70s. I bought the first version sometime late '70s. Lost them in a move (I still think my roommate stole them...) Replaced them with the 2nd, better version which I've had since then. They are my reference for comparing all headphones and even when evaluating loudspeakers.
I've bought & sold probably 25 or more headphones since then (top-of-the-line BY, AKG, Koss, Senn, Grados, etc.) and have never found anything close to what the Stax Sigma can do.
There are some that have better/more bass but that's it. These beat on everything else and are absolutely unique even compared to Stax's more recent product line because of the ear speaker design/philosophy of front-aligned electrostatic drivers which use YOUR pinnae to channel & modify the sound.
I did my Masters thesis on "general systems theory applied to human psycho-acoustics" — most testers, reviewers & musicians don't realize that, above all other factors like equipment quality & recording techniques, the biggest factors by far in SQ is your room listening space, your position and YOUR Pinnae, Cochlea & Auditory Nerves!
No two people Ever HEAR the same signal in their brains!
The Sigmas are the Only headphones that make use of these principles and thus sound most realistic. The only thing lacking is lower bass extension...simple physics means you can't use a 2.5" diaphragm to substitute for a 20" kick drum! But the bass is Fast with the best rise & attack & decay of any headphone OR loudspeaker.