12au7 tube?

Well why not try the Telefunken 12AU7? CBS 5814 are good as are the Amperex. Really depends upon which direction you want to take the sound. I always found the RCA clear tops primitive and bright sounding. 

I am pretty certain that the French tubes, La Radiotechnique (RT) were actually made by this factory which was owned by Philips Europe. Has a unique acid etched code. 

marco,

The 3 types you mention above are all Philips Europe companies. Made in Holland, Belgium and France respectively. Each has a unique acid etched code which begins the second set of each code. For Philips/Amperex is was a delta or triangle on its side, Mazda a L and LA Radiotechnique a plus sign directly over a slash or minus sign. All have many internal similarities and all have the 4 seams coming out from the nipple on the tube tops. There is much online about these codes and is the only true way to identify a Philips Europe tube and I have never known these codes to be faked. Many of the Holland made tubes will be branded Philips as well as Amperex.

You speak as if these tubes are easy to counterfeit. Just look to construction and the acid etched codes. RCA black plates are fairly common. Mullards are among my least favorite 12AU7s as are the RCA clear tops.

Well these are called life tests and should be done on all tubes sold. Most likely they are not by many sellers, especially on ebay. Having said this, a very well respected tube seller advertises "new old stock in white boxes", which translated means white boxed tubes testing as typical new old stock. Not the same thing. The same seller claims that Sylvania Bad Boy 6SN7GT varied in construction and included 2 and 3 hole plates as just on example. 

Best way to tell if a tube is NOS is by a close inspection of the pins. Deep grooves or burn marks are signs of use as is baked on lettering which cant be removed easily with a thumbnail or a wipe. Original boxes arent a definitive sign of NOS condition but they are a good start. Some tubes, like JAN tubes, were bulk packed without original boxes, but this is fairly uncommon in my experience. I have had used tubes which tested higher than actual NOS. Not unusual for true NOS test levels on the same tube type, from the same manufacturer from the same production run to vary up to 20%. So typical NOS levels listed on some testers are averages which arent particularly precise.