^^Yeah, again what priscilla said. I've never tried the WA7 without the tube power supply. I wanted the option to change the sound signature of the amp so I went tp right away. Plus I got both for 1100 from a guy here on head-fi so it was a win win. If you shop around you can probably find a similar deal on a used one. Isn't groovyd selling his now?
Anyway, as far as why the power supply makes a big difference in sound, the only reason I've been able to somewhat grasp is it's the way the different tubes discharge then bounce back from a voltage drop via power pull from the amplifier section. I actually asked my electronics class professor in college about this exact thing cuz I just bought the WA7tp during that semester. He started drawing stuff on the board about jumping electrons, heater elements, bias, diodes, triodes, etc, and I was like uhhhh ok so basically tube magic right? He was like yeah pretty much. Lol. I tried.
But honestly the mystery behind those glowing glass cylinders to me is what makes them so freakin cool. The first time they lock you into the music just turn off the lights, stare at the glow, and let go of all that reasoning. Not sure if HD800s will do all that but with Audeze LCDs hours go by and you get LOST in the music. It's crack.
Also, never judge a tube amp's sound by what you hear the first 30-60 minutes from turning it on. Once they get really warm and hit their stride it's game on. You'll see.