Typically comparing digital outs be it transport or other media is all about jitter. It was shown some time ago that jitter is the sole cause of the primary auditory differences between transports / digital media. In fact, a good low jitter USB DAC can sound just as good as a very expensive and presumably high quality transport. The trick with transports and often other digital media is telling which clock to be the slave and which the master.
So in the end it is more about having low jitter than having the best gear ( i.e. expensive ). I did a pile of tests with transports against my USB DAC ( Grace Design m903 ) and honestly heard little to no difference in the test tracks going between sources. I was pretty surprised when I fed my ultra cheap Sony Discman into the DAC via digital out and heard again little to no improvement.
I would say if you hear little to no noise in your system at high volume and no music playing then your DAC is at least low noise. Without actually measuring it or comparing it against other systems I have no way of telling for sure, but if you are satisfied with the DAC performance perhaps look elsewhere in your system for improvements.
Headroom tells us:
"As mentioned above, the source is generally (keep reading!) the most important aspect of an audio system. We build a variety of digital to analog converters (DACs) to further improve your audio source. HeadRoom amps with internal DACs, and our stand-alone DACs, both contain high quality DAC chips in our designs; and, these contemporary high-end digital recievers and DACs are extremely good at reducing or virtually eliminating jitter from the digital bit-stream. Now, here's the exception to our rule about sources being so critical: when constructing a front-end for your stereo system, it is much less important where the digital signal is coming from when a very high-quality DAC is used. We've even found, in fact, that sometimes very expensive CD transports sound only slightly better than the digital output of inexpensive CD players or USB audio outputs from computers when using a good DAC. So, keep that in mind when putting together your rig!"