Which 6c33c-b tubes for BAT REX monoblocks ?

I asked that very same question to the local authorized New Sensor dealer (vivatubes), and they told me there is no new production of the 6c33c variants. They’re just rebranding the NOS Soviet stuff.

Interesting.  Thanks for the input.    

That's the sign that the tubes are not matched, all you have to do is to rematch the tubes, if the tubes are out of spec that will also cause the circuit to shut down making the tubes unusable.  The problem will be to get someone to match the tubes, the best way of matching the tubes is to match them in your amps, but which may also require adjustments.  So never throw away your used tubes they still may be good to go just that you may have to find another matched tube, and always buy them in matched pairs from reputable sources.