I asked them months ago and their answer was:"We do not get involved in the aftermarket tubes, we just know what we sellworks, other brands may have problems holding the voltages"
Yes but IMO a pair of Raytheon 6SN7GTB can blow away any russian 6SN7.
And unfortunately almost all NOS 6SN7 are expensive. So, in a fear of a problem because of voltage, we (BAT owners) must know which type of these 6SN7 can handle 500v or 600v...?
It can't be a secret!