Bass Guitar distorted at low levels

[draudio801]

I just got my Spark to use as a practice amp for bass guitar. I am having difficulty getting clean level from the bass while listening with headphones (Sennheiser HD600). With the Master and Output at 10 any setting above 9 o'clock on the Gain results in distortion on the low end. Unfortunately, the level in my headphones at that setting is so low I had to check to see if they were even on.

Music level from my phone over Bluetooth plays much louder and is clean.

How can I get more clean level out of my bass while using headphones?

Thanks!