KoshNaranek
Headphoneus Supremus
Schiit's response was basically noncommittal: they couldn't recommend any tube not specifically listed as compatible and use of the 6H30PIs would be at my own risk. At any rate, I tried them and they seemed to work fine. My ears are not nearly as educated as many here, but I heard no improvement in sound quality (which I did when I used them in the Little Dot), maybe even a little worse in soundstage and separation. What did concern me a bit was how hot the Valhalla 2 got after several hours. After I put socket savers in (almost entirely for aesthetic reasons; I think the amp looks nicer with the tubes sitting up higher) I noticed the Valhalla 2 ran noticeably cooler--quite warm but not borderline hot as it did without the socket savers. With the 6H30PIs in, the amp ran pretty hot again over several hours (though always touchable). Since I didn't hear any sound upgrade, I just went back to the stock power tubes (and the Valhalla 2 runs cooler again).
Thank you for the update