Just my thoughts on this. Some depends on what tube they use in the mic. Years ago, my first tube mic was the MXL V69 Mogami and that used a 12AT7, which can be found most anywhere and from several manufacturers. I purchased a pair of NOS Mullards from an eBay seller and swapped out the chinese tube and it did get better sounding. My current "go to" tube mic is the Miktek CV4, which uses a NOS Telefunken tube(can't remember what the number is). I asked Miktek if I should purchase a few spares and they replied that they had stock to cover probably 20 years so I shouldn't worry about it
I am a firm believer in having spares, so maybe buy 1 or 2 from WARM, just in case. Most tube mic specialists that I have spoken with have said the tubes should go 10 years easy as they don't see much abuse in a mic(all bets are off if the mic is dropped). Some tubes are soldered in and some are in sockets. I would open the WARM mic and if it has a socket, then replacing it yourself is probably going to be fine. If its soldered in, I would send it back for repair, but I know many that would still tackle it themselves
