WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
It might not be an imbalance anyway, as Mullard was part of Phillips so their valves were made in each others factories. You can buy Mullards made in Holland, or Philips made in England, or Valvo (Germany) made in France etc. Some 'Mullards' were made in India or Japan.
If you are concerned about manufacture then you should ignore the brand printed on the outside and look at the works on the inside, and the etch code. Even then you may, or may not, want to have matched valves. Matched does not mean same factory code but same characteristics - some ebay sellers get this wrong (through greed or ignorance, not sure).
I am a Luthier of 30yrs & want to try a few things regarding tube amps & harmonic content, a lot of the amps fall short in the way of harmonics without a truck load of electronic gadjets to make them sound good.