It sounds to me like the amp is oscillating when power or treble is applied, but only on one tube?!? Odd. Perhaps a bad supression cap or a bad grid resistor. It also occures to me that if the bias resistor has failed short that there may be too much bias voltage on that socket. Also, if that socket screen resistor has failed that would stop current and cause the high reading as well, but it wouldn't seem that it would only happen at high volume or treble settings?!? Any other symptoms?