I received several reports of Visseaux 6N7GT failing. Up to now 3 people who used these in my circuits had failures, one person had 2 tubes going bad, that is a total of 4 failed tubes.
This does not seem like a coincidence any more. I have zero reports of 6N7 from other brands going bad. I never used the Visseaux myself so I have no own experience with them. I'm just relaying what I hear from others. Failures happened in The Octal Preamp Mk1 and in two different power amps which use the 6N7 as driver tube.
Apparently they tend to develop heater to cathode shorts. In all my designs the voltage between heater and cathode is far below the maximum allowed. Also plate voltages and dissipation are within the allowed limits.
Those people who used the Visseaux all liked their sound a lot, but with these repeated reports of failures I'd discourage everyone from using Visseaux 6N7 in my designs.