The pre-amp valve ratings are largely meaningless as they don't seem use any defined parameters — every vendor seems to use a different "rating" scale.
Most of the of them seem to be some sort of "gain" rating, which are done with the pre-amp valve in fixed bias, so this really doesn't have much to do with a cathode biased pre-amp valve in a real amp.
The gain of many stages in valve amps are determined predoninantly by the passive components and not the valve itself, and regardless I'm not sure if you could hear a 5% increase in gain in any stage one stage.
The main concern regarding pre-amp valves is that they aren't excessively noisy or microphonic, so this is worth testing for although, obviously, there is no standard as to what is "acceptable".
