In my opinion, don't. It might be decent with an HE-400 but it is absolutely horrible with my Fostex T50RP (low impedance ortho). You'll be better off looking somewhere else.
No idea on the 8x factor though.
A DAC changes from digital to analog, as you've said. But each tube is different and has an effect on the sound, this effect is called colouration, because the tubes add something that wasn't originally there. A tube changes the sound as it flows through it. Since it isn't a tube amp, the sound should isn't affected by the tubes in an amp(From what my limited research tells me)
Colouring has nothing to with how can a tube in a DAC colour a sound, even an attenuator or a cable can cause colouration imo.
Mind you, I might be wrong.