To get right to the core of your issue: You do not mention how many tubes failed, how exactly they failed (blew up? got noisy? frying sound...?) and whether they were all from the same shitty batch, or whether they were widely sourced Telefunken NOS versions. Read this stickie, one of three I wrote, for an idea how hard it is to select microphone tubes: http://repforums.prosoundweb.com/index.php/topic,1141.0.html
The problem for me is that this circuit is somewhat unusual in its layout and its values used. I would probably have started out by using a n EF86 with a 6.3 V heater, a cathode bypass resistor which would dial in about 1.6 to 1.8 volts, and a plate resistor/voltage divider that gets me around 75V plate voltage. I would even try that set up with an fresh and proven healthy EF800. I am not familiar with a situation where any of the components you are using could deteriorate and destroy the tube, as you have been experiencing. It would help an awful lot if you had a friend who also is using this unusual tube in a condenser mic.A friend of mine, Scott Hampton (scott@hamptone.com) experiments with DIY condenser mics and is quite knowledgeable. You may want him to read this thread.Best of luck,KH
P.S., I was told that my microphone malfunctioned again today, in very short order.