Likely gradual unless a tube starts going bad. You will know when that happens. Most hybrid amps have tubes in the input (which last a long time) and solid state outputs. Check out Brent Jesse or Upscale Audio for tube options. I would wait a while before jumping into other tubes, so you can establish a baseline that would make it easier to identify the characteristics of the other tubes.
Tube life
Hey guys/gals, happy Labor Day weekend! Newbie here. I recently purchased my first piece of tube gear (the McIntosh hybrid integrated). I’m loving it, even with my power hungry Maggies. Anyway, will I notice a marked degradation in sound as the tubes age? Or is it a gradual glide path to less-than-Nirvana sound? Thanks for your comments! Any suggestions on tubes other than stock?