Question on NOS tubes bias doubt

The tubes could be matched for triode balance and trans-conductance but not for gain. In this case, one tube appears to have higher gain than the other. Just use the balance trim to center the image and enjoy the tubes. That is why its there. On my Cary SLP-2002, I have to adjust the filament voltage on the circuit board with a meter, fidling with a small blue pot. Be glad the SLP-05 has the convenience of just turning a knob on the faceplate.

"Transconductance" IS how you measure the "gain" of an electronic device(tube, op-amp or transistor). You'll note that it is determined by finding the difference between the voltage at the input and the current at the output of the device in question: (http://en.wikipedia.org/wiki/Transconductance) Tube pairs with matched transconductance are sought out to avoid system imbalances. If the tubes in question were matched as to triode balance and to one another: there should be no imbalance from channel to channel as a result of installing them. That's providing there is no imbalance elsewhere in the system that the installation of properly matched tubes happened to reveal.

Thanks for rodman99999 info, I further confirm one of the pair has problem because that one started producing noise after couple of days trial.

The tube becoming noisy that quickly also confirms Mr N's response regarding it's being about to expire.

True, Newbie concluded that precisely. Now that the seller tell me to send back to him for refund or replacement. Thanks all.