Update: Put 2 Sovtek/EHX 12ax7s into the v1/v2 position and nearly pissed myself with joy as soon as I turned the amp on again. Sounds fucking awesome now. Nice and tight. Not 5150 tight. Very unique. Without a screamer and the stock speaker I can get tones ranging from Kvelertak/Mastodon (the stock speaker has a fat and non-fizzy midrange tone perfect for those rumbly, grindy Kurt Ballou tones) to Porcupine Tree-style crunchy cleans. With the Tubescreamer in front it definately covers a lot of metal. It reminds me a bit of that Engl Darkane used on their newer albums (Savage?). Very charcteristic high mid range when you crank the treble with the stock speaker. With celestions it really can do a lot of modern tones. Not compressed enough for those chugga chugga squeal squeal deathcore tones, but perfect for more classic marshall-ish highgains. I ALMOST got a Djent tone out of it by turning the tone up the screamer and cranking the treble while keeping the presence low - wasn't really saturating enough though. It's difficult to get nice sounding pinch harmonics out of this amp.
I'm so glad I made this tube change. The thing sounds like a different amp now.
So, anybody wanna buy 6 JJ ECC83S for 60 euros?