My headphones are JVC HA-S680's:
https://www.amazon.com/JVC-HA-S680-W-White-phone-Japan/dp/B00FS0DFTK
I'm not trying to improve the sound quality.
I just need to increase the volume somewhat.
EDIT: I have desktop speakers hooked up to my TV, and the volume going to the headphone jack is anemic.