Hi San jose. Hey, Merry Christmas.
Is your HF-81 circuit still stock?
It is always advisable to replace the el84's with closely matched pairs/channel with any self biasing type amplifier.
Randomly substituting output tubes in an amplifier employing a fixed biasing circuit is just asking for trouble, particulary with a 60 year old amplifier.
It would be in your best interest to check the value of the cathode bias resistors in your unit before swapping out the old tubes.
After close to 60 years,the original resistors have more than likely drifted away considerably from their original value.
The original cathode resistors were 165 ohms [rated at 5 watts] and should meaure within a 5% tolerance.
Furthermore: When substituting old stock el84's for the modern Russian equivalents. I would highly recommended increasing the cathode resistance to 180 ohms [rated for 5 watts]