However if the valves are too far adrift, then they could have very different plate dissipation values (unless you compensate for that within the circuit by allowing for individual bias adjustment so they match within normal tolerances), and so quite different power outputs. Which means that the mains hum on the DC to the power valves isn't cancelled out that well, so the amp could put out a lot of hum. You can counter this by better filtering of the DC to the power valves, but this isn't standard.