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A newbie tubie but, my first amp uses the 6BQ5 (EL84) and after doing a little research - there is a whole world of tube swapping opportunities out there!

Seriously. Isn't it fun? I'm just getting into it with the Moscode 401HR. I started with a pair of 6DJ8s.

The 6DJ8 was designed to be an ultra low-noise wide-bandwidth tube for the preamplifier modules in Tektronix oscilloscopes, which were the Rolls-Royce of oscilloscopes in the 1960s; they had no competition (Hewlett-Packard always tried, but could never catch up).
Until recently, the audio preamps from Audio Research and other top manufacturers were using the 6922/6DJ8 in their active gain circuits, but now that venerable design has been surpassed by the 6H30. This is a tube that Russian engineers developed for critical aerospace/military applications, long after the USA had abandoned tubes for solid-state and integrated circuit designs. It would seem to be the current champ for low noise and signal purity. BAT and Audio Research are certainly using it for all of their best preamps at present.
The Russians kept tube development active until the 1990s, while the rest of the world pretty much decided the tube was totally obsolete around 1970 (except for high-power transmitter tubes for radar and commercial broadcasting, which are still current in industry).

The Russians used tubes in their military gear because tubes are not effected by an EMP. An electro magnetic pulse (product of a nuclear explosion) "fries" solid state circuits. It is possible to "armor" solid state devices to protect them from EMP's but it is difficult and expensive.

Actually, EMP's do not usually "fry" solid-state devices; what they actually do, though, is during the EMP the solid-state device changes from a semiconductor to a conductor; a big difference in conductivity which wipes out your data if not caught instantly and prevented in logic circuits.

When I worked at Autonetics in the late 1960s, My job was to perform final testing and certification of the Minuteman II ICBM missle flight guidance computers, which used RTL and DTL logic circuits.

These used the first production integrated circuits in the world, and they were not real sophisticated in their architecture by later standards: voltage sensitivity was a major issue anyway with them.

The way these computers dealt with that issue was to "dump" all critical data for current processes into a shielded hard disc memory during an EMP or high radiation event (triggered by a special sensor). During this "dump" event the voltage regulators for the twelve DC power supplies would drop most power supply voltages to about 20% of normal and restrict current to most circuits. After a 10 millisecond delay the system would check to see if the event was over, and if it was then it would begin a recovery sequence and go back into normal flight guidance operation.

And to think NOW the Russians through Sovtek, Svetlan and others are just making music through Marshall, Fender,KOMET,FUCHS, THD and others amps. those swell Russians. Who makes Electro Harmonix, Groove Tube, I found a bunch of NOS Philips (Dutch and U.S.A Amperex) GE U.S.A., www.partsexpress.com has a nice selection, at non audiophile gouging of prices