@Kapazza
Would you mind sharing a comparison of the MJ2 and Folk?
I have an MJ2 and almost ordered a Folk. I am torn on going Folk, Ampsandsound, or DNA, if I even stay with a tube amp.
Like others said above, my preference between the two depends on so many factors...acoustic guitar with vocals, or symphonies? FV > HD800S all day, every day.
Anderson .Paak? MJ2 > Arya.
"In between" stuff (meaning a mix, a bit of both, more of an even spread on the frequency spectrum), such as rock/country...depends on which headphones I want to use. But the Arya sound much better to me on the MJ2 than the Folkvangr, whereas the HD800S is just better on the FV while still great on the MJ2.
If you ultimately want to cycle through a bunch of different headphones, you'll need to hang onto the MJ2. Like most tube amps, the Folkvangr isn't going to allow planars to show off their full potential. Not really a detailed comparison like you asked for, but hopefully that helps.