From my personal experience, I can tell you that tubes, being used in the quantity that they are, get used up by the manufacturer. As they replace the supplies they need, if a company making those tubes is currently turning out less then satisfactory product, the amp manufacture may find a different manufacturer for the tubes needed, rather then put less then satisfactory tubes in his amps. Therefore your amp may have a better tube in it at the time it was made, which may come from a different company, if the old tube company was not making great tubes at the time your amp was made.
If your amp is running well and sounds good, the least of your concerns should be what the company decides is the best tube at the time your amp is completed to use to make your amp sound it's best when it leaves the company.
This is not meant to be a rant on you, but only to describe, what I feel is a valid reason why you may have a different tube in your amp then other amps coming from the company at the apporimate time your amp did.
I've played Diezel amps since 2002, and whatever Peter decides to put into my amp as a tube from a given company is fine with me, because the amp always sounds great. Don't worry about the tube, just enjoy the amp.
