I have both and enjoy both, but ultimately I think the WA8 is better, albeit less portable.
The WA8 is warmer in the low end with it's stock DAC, which one may prefer or not. I often use mine with an external DAC if I'm looking for slightly more linearity.
For actual portability, there's a good argument for the Tube Mini as it's simply much much smaller and easy to carry around. While I move my WA8 around, I tend to listen in more static locations, whereas the Tube Mini is good for actually using while traveling.
Both the Woo Mini and WA8 handle high impedance DD cans like you want with ease. The HD800S and WA8 are an incredible combo. Wide, deep soundstage. The onboard DAC is pretty warm tuned as well.
Like @AnyDave , I use.an external DAC as well. Especially with the Mojo2, I get all the tube goodness, but better detail and (I think) better overall timbre than the onboard DAC. Portability is not an issue. Neither the Mini or WA8 are good on the go devices. I've taken the WA8 on vacation several times. No issues.
The Mini is a very good dongle DAC, and it does come close to the WA8 in sound. Ultimately, I use the WA8 90% of the time since I can use it as just an amp and is much more usable with my easier to drive HP. The Mini balanced out has incredible gain being a hybrid design and I end up at the lowest 2 volume settings to drive cans like the Utopia or Charybdis.
TLDR; If you have the means to buy the WA8, just do it. It is an underrated and wonderful portable SET amp.
Many thanks both for the answers!
I think I'm confirming I should go for the WA8...
Thanks!