The two 6BQ5/EL84 output tubes in the MG Head DT and MG Head OTL amplifiers, including the Mark III, do not have to be matched as this is not a push-pull amplifier. Each output tube operates independently as a single-ended triode (transformer mode) or in triode mode when used with the OTL output. The 6BQ5 is a pentode tube, but in the MG Head it is used as a triode. The additional tube grids are not used in this application.
As long as the tubes test in or close to the "good" range on a tube tester, have no internal shorts, and are not microphonic, they will work just fine.
Only twice have I experienced problems with output tubes in the MG Head. Once when I tried a tube that turned out to be highly microphonic. The other time was when I refused to accept that a particular pair of Mullard tubes that showed no signs of life when tested in my tube tester were too far gone to use to power my headphones. They were far too weak even for the limited power requirement of the MG Head. Deadest tubes I've ever seen.
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There ought to be a law
Against you comin’ around
You should be made
To wear earphones
Because something is happening here
But you don’t know what it is
Do you, mister jones? - Ballad of a Thin Man, Bob Dylan
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