Why do NOS Tubes sound better than new tubes?

Lots of manufacturers back then A lot of competition
Now only a couple plants make tubes. No competition

Not all NOS tubes sound better than new ones. The ones that do are because of the reasons cited in the above posts.

Agree with Tpreaves. I've replaced Mullard xf1 EL34s as driver tubes in my SET monoblocks with current-production Genalex Gold Lion KT77s because they sound better. OTOH, NOS tubes are often preferable. But it's not a sure thing and always worth trying the new ones.

I heard a story about an RCA tube making machine (whatever they are called) that was new in the 20s and made great tubes but in the 30s the fittings got looser and the 40s looser yet etc. etc., each year tube quality went down due to worse tolerances. They kept the machine for several decades so tube quality at the end of the machines life was relatively poor.

One interesting explanation...

They kept the machine for several decades so tube quality at the end of the machines life was relatively poor.

Could explain why there are so many complaints about new records people have to keep returning.