Manley Jumbo Shrimp Tube issue - where to buy qualtiy GE 5670W tubes

Perhaps this should be posted in another topic but I wanted to start here.

I am having issues with replacement GE JAN 5670 tubes for my Manley Jumbo Shrimp preamp. I had the pre chassis tested and it checks out fine. I am concluding that the replacement tubes I have bought are consistently faulty (low volume after warm up). I have one good 5670 and it works fine for hours in either channel. 

Anyone else experienced issues with the GE 5670W tubes and/or can recommend a source that has presented no issues? The sellers I have worked with are more than willing to exchange or refund for faulty tubes which I appreciate but I just want to find tubes that work.