A tube tester won't apply the full B+ voltage testing a tube. Even the top testers won't if the user doesn't know all the operating parameters of your amp.A lot of newer amps drive tubes a lot harder then they used to back in the tube only days. Vintage amps also drive tubes harder now do to the higher line(outlet)voltage than we had years back.That might explain why good NOS tubes may be failing sooner than they should be.If a tube does flash in a amp,I wouldn't want to reuse it even if it wasn't at fault,assuming a part in the amp failed causing the tube to flash.