No Doc, I haven't. However I have considered putting a +ve DC offset on the entire heater circuit to reduce the potential failure of V2b.

In this and most other designs where V2b is a cathode follower the voltage on the cathode is often up in the high hundreds with 190VDC being typical. The limit specified by most ECC83/12AX7 manufacturers is 200V maximum between heater and cathode, so most amp designs are pushing this potential failure point for nil other reason than simplicity of design and reduced component count. Adding a simple voltage divider to the centre tap of the heater windings and raising the DC level to around 30 or 40 volts DC evens up the "stress" on all the ECC83/12AX7 tubes but most notably on V2b where in most normal designs if an 'arc out' were to occur and apart from possibly irrevocably damaging V2 it will also send a rudely large and potentially lethally damaging spike to the speakers... And given the quality of most tubes today it is a cost that I'm not keen on enduring.

A comment on "Quality" - Tubes have never been 'quality' items. Some people are lucky and their tubes can last a life time, others not so much. so you get good ones and bad ones and the brand has little to do with it. In the last 12 months I've purchased near 2 dozen tubes of various types and have had 2 fall short for different reasons. So on average for me that works out at about a rough guesstimate of a 10% failure rate from new, which is far poorer than what the solid state cousins can account for. It is what it is, but if I can save risking $400 or $600 (or AU$800 for a new pair of Alnico Blue's) worth of speakers by doing what costs me a $1 modification then I'll spend the $1.