Need advice on Unico 12AU7 tube rolling

Don't have one brand for you to try, but these tubes are not expensive in the world of high end audio. Check a place like Triode Electronics in Chicago (great source for tubes & related parts.) For about $200 you could buy a pair of every 12AU7 they offer (8 pair).

To replace tubes, just turn the equipment off and give it 10 or 15 minutes for the tubes to cool completely. Pull the old tubes out of their sockets (gently with not too much rocking) and then insert the replacement. The tubes can only be inserted in one orientation, so look carefully at the alignment. Replace the cover, fire things up and listen!

As noted by another poster, "NOS" stands for "New Old Stock." These are tubes manufactured years ago by well regarded companies, many who are no longer in the tube business. Some people regard these old tubes as superior. In fact, there is a dirty little secret in that some dishonest people are actually relabeling modern tubes with old labels such as GE, Mullard, Telefunken and so on. Buy NOS tubes only from a reliable source.

(Tubes have zero deterioration in storage as long as they are not physically damaged. A 50 year old tube that has never been used is as good as the day it was made.)

Many well known manufacturers of tube got out of the business years ago and almost all modern tubes are now made in Russia, China, Slovakia and the Czech Republic. There are a very few American made tubes but they tend to be specialty, broadcast or military items.