Below is text I got from CJ's FAQ page.
Questions about Conrad-Johnson Vacuum-Tube Components:
Q. Can I replace a tube with a different tube type?
A. No. You should only use the tube type that is specified by conrad-johnson or an exact substitute. Using a different tube type can be expected to degrade performance and may damage the product.
I would call or email them anyway. I have VTL 300 Deluxe and changed out the 6550's with KT-88's recently. The reissue Gold Lions KT-88's are as good as everyone is writing.
Good luck
Questions about Conrad-Johnson Vacuum-Tube Components:
Q. Can I replace a tube with a different tube type?
A. No. You should only use the tube type that is specified by conrad-johnson or an exact substitute. Using a different tube type can be expected to degrade performance and may damage the product.
I would call or email them anyway. I have VTL 300 Deluxe and changed out the 6550's with KT-88's recently. The reissue Gold Lions KT-88's are as good as everyone is writing.
Good luck