I'm curious: When you turn the volume all the way down, do you still hear the music?
Its been awhile since I've heard the WA7 but all I remember was I felt the DAC section was complete shit. Amp was just classic "Woodoo" sound.. but I can see it being a nice pairing with the energetic (fun) sounding TH-900. If that is the only headphone that is in question the WA7 has some possible value. Otherwise, I would stick with what you have, save, then buy something higher end (maybe a DNA Stratus.. as I loved the pairing with the Fostex).
The WA7 you heard was with the external cube power supply?
Even with the PS, the WA7 is still a class lower. Did some AB at RMAF with and without, not remarkable. Taurus would be on another technical level.