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 sounds more like a capacitor that is changing value.
As a tube ages, the emission will decrease. The effect will be lower gain and/or the early onset of clipping at high levels.
i should mention that the gain is also lower. it takes about 3 times the turn of the volume that is usually required to reach the desired listening level.
the whole component is fairly new and the caps should be well within their lifespan.
May be that the +B is too low because of a bad cap or a false contact in the heaters rail so the tubes take more time to get normal cathode temperature.
You might get more distortion at levels below clipping. It is conceivable that your ears may perceive this as a frequency response problem, but it isn't.