The original specifications of your amplifier is the best thing you can go by to verify its performance, but additional equipment is required. If the distortion, frequency response, power output, noise level, etc. is within factory specs, then it makes little difference what a tube tester tells you. If you don’t have this equipment, then your best choice of test equipment is what you hear. If your restored amp pleases you, then “GOOD” is good enough. In the world of audio freaks, this will put you in the “Golden Ear” category instead of a “Meter Reader”.

Vacuum tubes are not created equal. Manufacturers have tolerances, and any tube within this range will pass final inspection. If they were all exact, then venders would not be selling matched sets of new tubes for use with high end sound equipment. As you have found out already, your tube tester can indicate some of these differences, so about the only thing you can depend on from it is a comparison between one tube and another. Therefore, it’s not a bad idea to have spare tubes on hand for future use that can also be used to make these comparisons, which brings us to another subject.

It has been said many times in the past that the best tube checker is substitution. If you have another tube that is known to be good, then it can be compared in the actual working circuit, which is what really matters. In some cases, a tube that reads somewhat weak on a tube tester will work just as well as a new one, which leads us to yet another subject.

Vacuum tubes do not usually work at 100% in most circuits. If they did, then every tube in your amplifier would be operating at its maximum voltage, plate current, wattage, or whatever. Since your amp was designed to keep distortion below a certain percentage, just this reason alone is enough for the tubes to be operating well below 100% emission. In other words, distortion usually increases at higher tube outputs, and the designer of the circuit decides where to draw the line. This is usually a point where a somewhat weak tube can sometimes still be within specifications. Since the design perimeter of the circuit determines the operating conditions of a tube, this brings us to even another subject.

Tube testers do not tell you how a tube will be used in a circuit. Your tube tester supplies fixed voltages to the various elements of the tube under test, which may not be anywhere near those in the actual circuit where it will be used. In some cases, just a small difference in the bias voltage can change the characteristics of the tube in actual use. Therefore, a tube tester can tell you that a tube will probably work in a circuit, but it cannot tell you how well it will work.

So, what level should be the lower threshold of acceptability? Only you can be the judge of that.