I can't tell you if it's a DAC, but I can tell you about sound quality:
I use an ATH-AD700 with the MixAmp, which is connected to the computer through SPDIF. With Dolby Headphone, both music and games are fun to listen to. Notice how I said 'fun.' For games, this is nirvana, as the soundstage is great. However, music through the Dolby Headphone may be fun, but it is not accurate. That is expected because it is a DSP. It's not like it's bad, but Dolby Headphone was never made for hi-fi listeners.
Now, when you turn off Dolby Headphone... it's weird. I can't explain it, but I like plugging my headphones in directly to the computer's line out than use the MixAmp without Dolby Headphone. I don't want to say it, but the MixAmp (without DH) may, MAY destroy the soundstage of music. Someone more experienced may know more about what I'm hearing.
However, this could be psychological: I am always comparing DH to no DH, so the night and day difference may have made my mindset think that the MixAmp is bad without Dolby Headphone. Also, the mentality that the SPDIF is going into hardware that I only know is good for gaming may make me think that something bad is happening to my audio when it comes out the other end to my headphones.
So bottomline:
MixAmp is awesome for games, movies, youtube, anything that doesn't need high quality audio. Music, however... you may want to stick with your FiiO combo.