Ryllau- NOS stands for New Old Stock. However, you'll find many NOS advertised that are used- some tested, some not. In general, this means tubes that are no longer available from the specified manufacturer. Most current production is limited to Chinese, Russian, or Eastern European, with only a couple of exceptions.
Are NOS worth the price? That is a very personal preference, equipment/system dependent question. In many applications, selected NOS are considered better. They certainly sound different and the level of difference depends where in the components circuit the tube functions.
For the above mentioned tubes, I'd recommend that you check at Vacuum Tube Valley for their back issue surveying the NOS 6SN7 tube types. Do a search on the web for other sites as well.
I'm particularly fond of the Tung-Sol, Sylvania, and Ken-Rad VT-231 and Sylvania 6SN7W (especially the metal base version). I also like the Mullard CV series tubes. Your taste may vary. Good luck.
