The sts is a 600 ohm input & the output to headphones can be 4 to 8 ohms
you are going to need a transformer to ie: a matching one.

https://www.buckscom.com/catalog/in...p;zenid=c9c75091fd41393163587fc332951ec0

Or a Bogan WMT1A www.bogen.com/products/pdfs/specialelectronicspdfs/WMT1As.pdf