The problem went away with time. I don't understand how or why, but I didn't do anything. It seems it just needed to burn in. The amp works and sounds good.
I also noticed that my 339s internals are not like the pictures that are posted here by Yuking, but those are all premium or reference edition I think.
Mine is only the standard one.
I wanted to post my impressions of the tubes I got, but I always postponed it, plus I also can't write about the sound in detail. Sorry about that, and the postponing.
I have these pairs of tubes now:
Svetlana =C= 6H13C
RCA 6AS7G
Ken-rad 6AS7G
These are not the best for me, the highs are too rolled off, don't like them. Keeping them because I feel they will be good with the HD800s that I am currently waiting for it.
RCA 6080: the first tube I liked, tho a bit too bright maybe. Perfect for gaming.
Just an example, with the RCA 6080 I could hear if the terrorists planted the bomb in CS:GO much much further away than with all the 6AS7Gs.
When I had the 6AS7Gs in, all my friends could hear the bomb ticking with their very cheap headphones, but I couldn't and I was like "What? Where is it??" Then they mocked me for buying an expensive amp and headphones and not even hearing the bomb lol.
Mullard 6080WA: better than RCA 6080, the best price/performance tube IMO.
Tung-sol 5998: Great tube! Better than mullard with a bit more bass and clarity.
Western Electric 421A: My favorite. Sounds (a bit) better than 5998. This is whats currently in my amp.
As long as I use the HD650, I will probably only use the 421A and if they die the 5998. They are the best, but unfortunately also expensive.
Now for the input tubes. I have:
RCA 6C5
Hitachi 6J5GT
Tronal 6J5GT
With input tubes it's more hard to notice difference.
I didn't like any of the above ones. They sounded a bit too "muddy".
RCA 6J5: The first input tube I liked. It is also the cheapest!
Hytron 6C5GT (dead after 1 day use) I liked these. R.I.P.
GEC CV1067: What I currently have in my amp. Sounds right. Never compared to the 6J5 though.
Now I'm waiting for the HD800 and I'm also upgrading my ODAC to the Violectric DAC V850 (that should arrive next monday). I'm curious how my tube preferences will change due to these upgrades.