NOS means New Old Stock. These are valves made in the past, sometimes decades ago (most usually ’50s and ’60s) that were never used, and are still new-in-the-box. 

This differentiates them from their currently manufactured counterparts.

There are some who hold the opinion that using NOS valves – particularly in a piece of restored vintage electronics – is somehow more authentic.

So on the one hand we have NOS valves imbued with some psychological value, and on the other, since these things are not always easy to find, the normal laws of economics and scarcity apply.

Consequently, you can pay some serious money for NOS valves.

Is it necessary in a hi-fi amplifier? Probably not, if sound quality and reliability are your only criteria. But if you want something unique and rare, go right ahead! The new valves are specified exactly the same as the NOS ones, so they will be truly plug-and-play. And the provenance of your NOS valves will add to the uniqueness and mystique of your amplifier.

The exception for this is for guitar amps. Since these amplifiers deliberately over-drive their valves, to get the pleasing distortion they produce, different manufacturers’ valves of the same type produce different distortion. So in that case, experimentation to get the sound you want can yield results.