Blown tube issue

Yes...I know how to set bias and have done it periodically like Manley suggests ...I just completely re-tubed in October 2020 so one blew from previous owner set (expected) and one blew from the new set (unexpected) 

To blow a tube in six months is unexpected, and unfortunate, but unfortunately not unusual.

"...Set Bias for 300mVDC measured across 10 ohm cathode resistor..."

That is not driven too hard for an EL-34. Some amps drive them way harder than that.

OP here is the real question, was the tube failure in the same position? If not, move on and understand, valves fail. If it was in the same position check the bias per position that ONE is to hot.. 300ma is not plus anything, that is MAX. That is every valve? or per bank.. 300 sound HIGH per valve to me.. Anyway LOWER is better for now and accurate is even more important..

Make sure your using a DMM, not an analog meter, they are not close enough.. Every year they are suppose to be calibrated if they are being used as a TOOL, for looks that’s different. Who cares.. Blowing tubes.. MAKE SURE... the meter is good and you're at least bias 300 or LOWER not higher..

Surging ON is a problem too.. Make sure you have a 120 volt maintainer for start up, no voltage dips. Soft start or pre start really helps too..(stand by switch)

I have the older Macs that are 117 vac MAX they don’t like 120vac. They will actually run too hot.. but on a Variac at 110vac.. perfect... Those Manley aren’t that old... are they?

I have VTL 300 deluxe (Manley design) they like 110-115. Above that they run HOT. 110 I can almost tolerate them in the summer.. but I can adjust the bias, the Macs I can’t.. ALL fixed..

Fruit for thought OP..

Regards

Good comments from all the above.
If failure was at same position, it could be random but also a cause for some concern.
Also monitor the bias more frequently than Manley suggests to check that your amps can hold it there. Safer not to exceed the value whatever the sound, a bit lower the better for tube life. Also a question to the seller of what caps exactly were changed and why. 
Tubes can go funny and not created equal. For me two failures in 1,5 years with no constant use is at the limit of unexpected. 

G