I'm pretty sure that if you don't run a matched pair of power tubes in your amp, it'll screw up the Output Transformer in some way. Since the two tubes run in push-pull, while one's pushing output, the one is pulling back, so it needs to be well balanced, or you could cause some damage. Single-ended amps can afford to have unmatched tubes, since they are usually Class A wired, running all the time, and if you have a dual single-ended amp, they both feed the OT, and the other tap of the OT is run directly to ground, whereas in a Push-Pull AB amp, one tube is connected to one tap on the OT, and the other tube is connected to the opposite tap.