I love my Sprout, but the headphone jack... looking for suggestions

I have really been enjoying my Sprout. I have had it for over a year and my music sounds great through it - My one issue is my Headphones sound so underpowered through them. I have some Grado Hemp Open Ear… they sound amazing through my Iphone… but loose detail and significant volume through the Sprout. Any relatively low cost solutions for enhancing the sound to my headphones through my Sprout? Thanks, G