The O2 will power the 500's aplumb.
The test unit JDSLabs so kindly sent to us was setup with the stock gain of 2.5x and 6.5x, which should fit both low and high efficiency headphones. If you plan on using it solely with in-ears, you might want to set the gain set to 1x and 2.5x. To put it into perspective: this little amp has more than enough power to get the HiFiMAN HE-500s singing at 2.5x gain, and even the notoriously hard to drive HE-6s had plenty of volume available on the 2.5x gain setting.
There are tons of posts from people using the 500's and the O2 and working very well.
I use (2) Odacs and a MSii+....I cant tell the difference between the 2 except for the 2.25 volts out of the MSii+....both awesome dacs and work well.
Remember this is a "low cost DIY" amp that was designed to prove a point....that great performance and neutrality does not have to come at great cost....
If you honestly dont like it fine....then move on to other amps and dacs to your "objective" liking...
all the best