c-j tells you that you should only use tubes purchased from them. They may test their tubes more than others but there is nothing unique about their tubes. You can buy tubes from other vendors and many people do.
Many people also tube roll in c-j amps and preamps and often find tubes that they prefer from manufacturers other than the ones c-j uses.
There may be a problem with the warranty if you use tubes not purchased from c-j so I would ask them about that. I don't know if there is any problem or not, but if you have to send anything back to c-j for warranty work make sure you have c-j sourced tubes in the gear.
Many people also tube roll in c-j amps and preamps and often find tubes that they prefer from manufacturers other than the ones c-j uses.
There may be a problem with the warranty if you use tubes not purchased from c-j so I would ask them about that. I don't know if there is any problem or not, but if you have to send anything back to c-j for warranty work make sure you have c-j sourced tubes in the gear.