Mismatched power tubes sound?

Bob. Not to worry about permanent effects of output transformer magnetization. Quality output transformars are made from fairly soft iron, and does not permanently magnetize, and if they did, driving the amplifier hard would soon demagnetize them anyway. The magnetization is caused by the unbalanced plate current of the output valves making the transformer act as an electromagnet. Gain unbalance between tubes, or valves as we call them here in Australia is another real concern and can to a degree be compensated for by adjustable AC drive levels. This was a trick used in tube type Cinema Amplifiers, and in the Modulator Stages in Tubed AM Broadcast Transmitters. We have used the same trick in a high quality tube KT 88 or 6550 Commercial Studio Monitor Amplifier. If you are interested, we can send you some details on this amplifier which we call the "Enigma", 1 because we do not know why it sounds so good, 2 we do not know why we went and designed it.

Matched Output Tubes, are Mandatory for good results in serious Tube Power Amplifiers. Individual Bias adjustment for each tube is also mandatory to allow the cathode current of each tube to be the same value. This prevents magnetisation of the output transformer. Magnetisation of the output transformer lowers your available Low Frequency Power Output, as the transformer will start to saturate earlier thai if it was not magnetised. The actual Bias Voltage applied to each tube should also be nearly the same. If this differs significantly between tubes, you have a problem with one or more of the tubes, (probably worn out). Despite tube matching by the supplier, tubes still differ somewhat in gain, so some form of AC Drive balance control is of immense benefit. Be aware that tubes wear out, and change their characteristics drastically as they wear. I would discard, or re-match tubes showing as little as 20% difference, 50% are throwaways.