I don't understand your question but that's a real nice sounding tube.
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The Ken-Rads are some of the best VT-231's out there, but like anything else: They're not always perfect. You bought a pair of unmatched tubes. A slight mismatch shouldn't be noticable. That you have to compensate with a balance control says they're out quite a bit. Did the seller offer any satisfaction guarantee? BTW: Tube "biasing" is another story altogether(not related to your issue).
If you are saying that the center image of your stereo systems drifts substantially with these two tubes that they must not be matched, despite what you have been told, I agree.
In fact I would suggest that not only were they not matched, but that the weak tube is close to dying. IME, other than an onset of noise or dullness, drifting is the hallmark of a tube which is about to die. IME, perhaps there are other reasons of which I'm not aware. I do not use this tube type.
Both Rodman9999 and Newbee understand exactly what my problem correctly, yes the image shift to one side 10-15%. Thanks for answering all my question precisely and confirmed. Now I have faith to speak with the seller and he sounds pretty responsible, I will post what the outcome is for future case reference to other members.
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.