Worth investing in? These would be my main (only) headphones.
They sound pretty good but are bulky and heavy.
I grew up on a pair of these my Dad had, and I have a pair now. I prefer the Pioneer SE series over them, more detail and bit more airy. Though they are a classic piece of gear, so that is half the fun.
I have the SS-10, a model below those. It's okay, but bass is a little flabby. I prefer the Pioneer SE-505, but that is an unfair comparison, as the SE-505s are two-way, and the SS-10 is not. Not sure how the SS-20 (two-way) would compare.
To be honest, I keep the old things just for vintage integrity and fun... my Beyerdynamics best them both, and are my usual listening pair. Headphones are one area in which the best ones available today from companies like Grado, Sennheiser, or Beyerdynamics easily best almost all the vintage models. A few vintage ones, like the best studio models from Koss, AKG, Beyer, etc... are still quite good and satisfying, though.
Stax Earspeakers --vintage or new-- are absolutely wonderful, if you don't need isolation from outside noise, don't mind seriously high voltage straddling your brain, and don't mind looking pretty geeky. Ditto for Jecklin Floats, IF you can ever find a pair!