While I prefer Telefunken and Siemens CCas to Mullards in my Jadis equipment, that is my personal taste and has nothing to do with the quality of the brands. I think all you're doing is changing the sonic flavor; by using NOS tubes you're also getting better-made tubes, so they should last longer. If you like the Mullard sound, keep in mind that it is different than Telefunken's and Siemens', and you may or may not like them better. I'd say stay with what you've got, since you like the sound, and spend the $300 on some music.
Aging changes the sound slightly, it'll get duller and less dynamic, but if they test strong it's worth getting used ones, at least for spares.
Answer to your last question is probably yes, but I wouldn't obsess over it.
Aging changes the sound slightly, it'll get duller and less dynamic, but if they test strong it's worth getting used ones, at least for spares.
Answer to your last question is probably yes, but I wouldn't obsess over it.