Hey, thanks for coming back!
I sympathize with how hard it is to pick headphones. I’ve been buying, auditioning, and rotating through a small collection for years now and I still don’t always know how to make a decision.
For me, the important things in a headphone are sound, comfort, and build, in that order!
You’ve narrowed down your decision to two exceptional models, both of which I’ve had the pleasure of listening to. The 598 is so close in sound to the HD 600, and excellent in both the comfort and build departments. I reviewed those here.
Never owned the 990, but was able to listen briefly to a pair. They’re basically a fully open DT770, and even more comfy thanks to a reduced clamping force. They have really sharp treble and really deep bass, so if you’re prone to treble fatigue you might hate them.
But honestly you could flip a coin and be totally happy with either pair. If it were me I’d just buy whichever one I could find cheaper! They both go on sale frequently.
This old SBAF thread has a good comparison of measurement graphs between the Takstar Pro 80 and the DT770. They’re shockingly similar! I think Takstar basically reverse-engineered Beyer’s driver design.
Thanks for coming back and continuing the discussion, I really appreciate it! :)