So I wanted to use my ES100 with my HD599's and thought I might as well use the balanced connector while I was at it. Like all my purchases lately, I went straight to Amazon. I ended up ordering this one and didn't realize it wasn't balanced going into the headphones until I realized that my music sounded like crap.

https://www.amazon.com/dp/B09NDMYY2R?psc=1&ref=ppx_yo2ov_dt_b_product_details

I guess this is just a PSA for others. Hopefully I didn't damage my ES100.