I will argue that you should replace both tubes. The ability to hold bias is only one aspect of tube engineering. The tubes' tone also changes with age and this ought to be considered. I would and have replaced whole quads of EL-34s for thid reason in part. So take the sound quality into consideration a tube is not just a power maker.
OK to replace one power tube for amp?
I have Joule Stargate Monos that use the Russian 6C33C-B power tube, two per amp. One of them isn't holding bias very well... wandering a bit. Can I just replace the one tube? Each power tube is individually biased in this amp and I've heard that matching pairs with 6C33s is just not something that happens so it would seem ok to buy 1 but I didn't know if having tubes of different ages was bad.