Marakanetz: Bias helps to minimize the "knee" in the gain curve of a device. The less bias that you have, the more apparent the differences in gain are between devices. I work with Class C amps all the time and the gain curve on these is more like a "gain slope" than a curve due to the lack of bias. They are either off or on and the transition period is NOT pretty.
As such, gain matching would be more appropriate for a low bias circuit than it would be for a high bias circuit. Otherwise, one device may be at full output whereas another device in the same circuit with the same amount of drive applied might just be going into the point of conduction. Obviously, the levels of output / gain / linearity / distortion at that point would be very different things between conductors with different levels of matching. Bias keeps them turned on so the threshold of conduction has already been passed and the "knee" in the gain curve is reduced or minimized. This improves linearity, reduces distortion, etc... The higher the bias, the deeper we are into the gain curve of the device and the less differences one would have in terms of linearity at various drive levels.
The reason that line level tubes last forever is that they aren't called upon to pass any current. The more current that a tube has to pass, the faster it will deteriorate or flatten out. This becomes even more true as frequency is raised as the bandwidth of the tube falls quickly as the tube deteriorates. This is why tubes that are "flat" sound dull, lack air and top end sparkle, etc... The bandwidth / high frequency linearity of the tube has been reduced. Sean