How did you manage to measure one already?
In that case there would be no point in even considering tubes. Solid state always outperforms vacuum tubes at the same price point when it comes to strict technical accuracy. The appeal of tubes is that it "improves" the sound through euphonic distortions.
The Apogee is very good, the main complaint being that it's a bit on the harsh and bright side, but that makes it well suited to the HD650s which have laid back treble. It's very expensive just to use as a DAC+amp though, if you don't use it as an interface for recording music.
Using it for recording was the first reason why I got it for my laptop whenever I was away from home. I've been using it with the 650s just as often for enjoying music though, and was curious if using just the dac portion from usb to the audio out into a dedicated headphone amp would make any significant difference in sound. If not with the 650s, then maybe with the next headphone I get, whichever one that may be. I'm going to a meet for the first time next weekend, so I'll know what headphone my preferences will lean me towards after that, maybe. Thanks for your advice!