The headstreamer doesn't have a volume control - it depends on the PC's volume control. The E07K has a volume control. In addition, the headstreamer is a USB device only - it cannot work as a portable amp, while the E07K can do both.
The HRT Headstreamer has been discontinued and replaced by several other HRT products. The Headstreamer sold for approx. 30-40% more than the price of the E07K.
I'm not sure about the audio quality of the headstreamer amp - I suspect it was at least as good as the E07K and probably a bit better. My guess is that the difference is not a HUGE amount.
A better comparison is probably between the Fiio E10 and the Headstreamer. Those are both USB-powered dac/amps.