I have been immersed in tube audio for 25 years and in that time (mostly early on) I've bought dozens and dozens of NOS and OS tubes. I've learned that fetishizing any particular tube is pointless. Given that we all hear differently and have different systems, no one tube is going to be guaranteed to sound either great or horrible. It all just depends.
Yes, objectively there are better built tubes that will last longer than lesser built tubes, but what's the point if you don't like the way they sound to your ears in the equipment you have?
It's fun to tube roll, but with prices getting so crazy for the vintage stuff, I don't think it makes sense to spend more than, say, $50-$75 for the kinds of tubes we're talking about here (6AS7s, 6080s, 5998s, 6SN7s, 12AU7s). Chances are slim you'll hear a sonic difference commensurate with the cost. You might get lucky and buy just the right vintage tube for your sonic taste (and system synergy), but the odds are against it. I have boxes and boxes of vintage tubes that, although they're fine tubes, they didn't quite give me the sound I was looking for.
It's worth remembering that posts about vintage tubes on the internet from 20 years ago are from a whole other era: tube audio was still a very small niche market in audio (at least in the U.S.), and the availability of desirable NOS tubes was huge compared to now. It was so easy to buy tubes then that are now nearly extinct, and often for as little as $10-$20 a piece. It was cheap to tube roll all sorts of tubes from the 1940s and 1950s.
But that era is gone, and so when we read a 20 year old post on some audio forum that insists we just have to hear tube "X," keep in mind it was written during a time when it was relatively cheap and easy to do so. That window closed fast (by the mid aughts, as I recall).
Being more specific, vintage Tung Sol 5998s are a well made tube. 5998s have lower output impedance than a 6AS7 or a 6080, so with headphones like many Sennheisers, they sound brighter and faster. If your system sounds a little dull and sluggish, a 5998 might be just the thing. But there's no sonic magic lurking inside its glass.
I listen through Sennheiser HD800s (which, granted, are brighter than the 6xx series), and I'm perfectly content listening to a (relatively) inexpensive '60s-era RCA 6AS7 as a cathode follower. I also have such storied tubes as a TS 5998, a TS 7236, and a Mullard CV2984, but none sound "better" to my ears through my system than the plain Jane RCAs.
