As this thread is about tube makes and models that do especially well in Backert preamps, I'm going to reiterate my recommendation of the--admittedly rare and costly--Amperex 7316. This is what Brent Jessee has to say about them.
Simply an amazing tube. Amazing both because it sounds so incredibly good, and amazing how rare it is today. The 7316 was made by Amperex in Holland, I have never seen this in any other brand, nor have I seen it made by Amperex North America. It is a medical grade/aircraft equipment grade super low noise tube, not sold to the general public. It is Amperex's answer to the ECC802S produced by Telefunken, and the 10M series made by Mullard. Most have the Bugle Boy on the label, although I have seen plain Amperex labels, and OEM labels such as Beckman Instruments. The 1950s vintage with the long plates and D-getters are to die for. They are among the best sounding and rarest 12AU7 around today. Don't miss the 1960s halo getter version of this tube, as well. They sound nearly as good as the D-getters and are usually half the price. Very few dealers ever have the 7316 of either type in stock, well worth seeking out and worth the higher price. We try to keep a few of each type in stock, but it is not easy. Better buy them when you can find them!
Unfortunately, it's all true.