Do valves burn in?

[Clive]

However we do posess long term reliable sensitivities to stress and fatigue which might explain why valves used for amplification of music continue to sell to people who consider this charachteristic significant whereas they are rarely used in any other kinds of equipment.

I would suggest that this sensitivity explains in part why people are prepared to stump up the extra money and ongoing maintenance costs for valve amps. YMMV of course..

I've used valves for year and have just switched my power amps back to SS though it's still SE. Maybe a more telling characteristic is low or no feedback.