This is simply a few follow-up Folkvangr thoughts, maybe as a data-point for newbies. Just one person's take as a data-point....not an argument or counter to any prior impressions:
For what's it's worth, for tube newbies who picked up Folkvangr, I would make a humble plea that this is not a rewarding amp to develop tube nervosa on. You will see chatter around rolling a roll that rolled a roll on a EC9874-XN with the orange labeling from May 14, 1974 when Eddie "Lunn Bun" McChesire was known to be the working the line after a steak-and-pie night and his knowledge and flatulence famously produced DA' BEST EC9874-XN outside perhaps the Bad Bugle Drummer Boys of August of 79'........but stop. The 6N1P and 6N6P are interesting, fairly unique tubes that were cranked out in a small handful of Russian factories long before landing as an audio/amp option.
For 6N6P's....from 00's era DIY amps to the Valhalla 2 in the teens.....I've picked up 6N6P's from the 60's, 70's, 80's, and 90's. I've found pretty minor differences....with quality control (or my luck) slipping most in the 90's versions. I tend to go with some sets from the 70's most often.....I don't find the famed 60's variants notably better. These were precision equipment-bound tubes made in a smallish number of factories.....be mindful of wild audiophile claims (and paying the wild claim tax via high prices) like "Gold Grid", Foton's are the best, etc. (like 70% of 6N6P's have gold-plating, etc). Absolutely do get yourself 1 or 2 change-out sets before availability or price slide further.....but in my humble opinion I wouldn't go nuts on the 6N6P's.
I like the stock 6N1P sound quite a bit. It's not without weak-spots.....but it's detailed, fast, and stages in an interesting manner. It's great for more aggressive genres like metal. It may not be everyone's cup of tea.....but like the 6N6P's above I have (by 6N1P standards) a wide selection of these tubes and find no particular outliers. The tubes Schiit provides with Valhalla 2/Folkvangr are pretty representative of the model......I have zero idea what people have in mind as the in-family alternative when they claim "SCHITT SENDS LOW END STOCK TUBEZ CHANG TO BEST 6N1P". Also like above, for sure get yourself 1 or 2 sets of matches pairs now.....but personally I wouldn't go wild on some perceived unique variant vs. bird in the hand while they are still about.
The 'good' news is...the 6N1P's are realistically swappable for the 6DJ8/ECC88/6922/etc. family. The 'bad' news is.....that is a pretty famous family of amp tubes that's had a couple decades of picking over with the requisite remaining high prices. There are some pretty decent new-make and still some very good NOS.....but they are relatively pricey and full of tube-sales hype to keep premiums up on remaining stock ("Bugle Boys", etc.) Totally my subjective opinion......I've found Folkvangr sonically rewards some of the less hyped Amprex from the 70's/80's vs. NOS Telefunken's or new Gold Lion's (nice on Folkvangr, but can get pretty bright depending on the mix). My simple two cents would be to try a reasonable priced NOS pair from a trusted tube vendor.....if you think you hear wild improvements then down the rabbit (and budget) hole you go. Side grade or interesting but not mind-blowing....grab two more available, much cheaper match 6N1P pairs and feel good.