What tube pre-amps pair well with Pass amps?

May I respectfully ask for the names of some pre-amps that meet AES48?

Well ours do of course. Also Backert Labs, if you get the version with an output transformer to do the balanced output. Most preamps that use an output transformer in that manner should be able to support the standard- and it won't matter if the internal circuit is balanced or not.

I have to disagree with Atmasphere on two points:
1. I have a fully balanced Pass Labs XA60.5. The cable does make a difference.
2. I have two high quality Cary preamps. Cary SLP-98P Formula 1 mod, which is single ended. Cary SLP-05, which is fully balanced, but not upgraded. The single ended preamp sounds better with the Pass than the balanced in my case. That is why I am upgrading the SLP-05, for better SQ.

This is actually a really good example of what I was talking about. The Cary does not support AES48- any preamp that has RCA and XLR outputs, and does not need a switch to use one or the other, very likely does not support the standard, and so you will hear differences between cables that can be pretty profound.

One caveat to my points.
My preamps are very close to the Pass amps; so the interconnects are short. With a longer interconnect run, the balanced connections may possibly be superior.

The benefit of balanced operation really does not have much to do with the length of the cable. The benefit can be there even if the cable is only 6 inches.

One thing about balanced operation is that it is meant to help reduce the effect of the interconnect cable. If you've ever auditioned interconnect cables and heard a difference you know what I'm talking about. Audio engineers frequently poo-poo the idea that cables can make a difference, but they are using balanced lines and equipment that supports the balanced standard so they can come that conclusion not realizing that they are talking apples and oranges.

But Pass Labs are balanced input, so if you want to get the most out of both the amp and not have the cable be a big system influence, your investment in the amp will be best served by a preamp that supports the balanced line standard (also known as AES48). Not surprisingly, most high end home audio equipment does not support this standard! That is part of the reason there are exotic and exotically-priced interconnects (balanced cables need not be expensive to sound really right). Its a simple fact that if the preamp does not support the balanced standard, the interconnect cable will be part of the hidden cost of getting the system to sound right.

In a nutshell, here is the standard:
1) pin 1 is ground, pins 2 and 3 are the signal; pin 3 the inverted form of pin 2

2) The signal does not reference ground (instead pin 2 references pin 3 and vice versa) as that can cause a ground loop. This practice also means that the shield of the cable is not used for anything other than shielding and so in this way is fundamentally different from single ended connections!

3) the preamp should be able to drive low impedances with ease and no loss of low frequency bandwidth. 'Low impedances' means less than 5,000 ohms.
If the preamp cannot do these things you'll be hearing cable artifact and you simply won't get everything out of the amp as a result.