Tube life

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.

Darn millercarbon , you can't say stuff I agree with. It is not fair:)

Nor is it easy. What I mean is, tubes vary so much from brand to brand and even tube to tube its very hard to say anything meaningful about them without someone else being able to say something completely different- and both of you be right!

I mean, I used to think tubes were really reliable as maybe only one of probably 30 or 40 had ever really failed on me. Then in short order I had two flare out quite dramatically- and one of them was brand new! 

There really is hardly any difference between tubes, not only between brands but even between certain types. KT88 is better to me than 6550C but not by much. JJ is better than Svetlana but by even less. But I'm comparing relative to something like a $150 Quantum Fuse or $60 set of BDR Cones or even something like running a demagnetizing track through your system. Those range from 3 to 10 times more important than the difference between tubes, which to me makes buying a new set when you already have a set a waste of money. But another guy might think a barely noticeable improvement for $500 is the moon and the stars. So both can be right and it is darn hard to say without being set up either way to get skewered.

So good to know. Thanks.

@ jrod68 OP

Any suggestions on tubes other than stock?

https://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html

http://www.audiotubes.com/12at7.htm

http://www.audiotubes.com/12ax7.htm

Since that’s a hybrid amp, I don’t feel you’ll notice the signal tubes degrade as much. 
If it was a complete tube amp, with power tubes, you’d notice a big difference when they degrade. 
I have an old sansui tube amp. When those 7591 power tubes start to go, you really notice. When you replace them it’s like you have a brand new amp. I’d tube roll your signal tubes because their is quite a difference in the sonic signature of those tubes. It’s also a lot of fun. Glad you’re enjoying your new setup. 

I have a Conrad Johnson PV2ar preamp (bought new in 1981), solid state Hafler 100W amp going through Anti-cables to '78 Klipschorns.
Due to the high efficiency of the K-horns, when tubes start to age I can hear it easily....sounds to me like "wind" noise.
When the sound of the wind, sometimes a hum, gets obtrusive I know it's time for a tube change.
Brent Jesse has been my supplier for a number of years now and his advice has always been spot-on regarding the sound of NOS replacement tubes. 
In my case there is a noticeable sound difference between tubes.....nothing overly dramatic, just a sweet richness that varies from brand to brand.
If your system isn't overly sensitive, and the audible tube sound differences are negligible, I would say it doesn't matter...... why spend the extra money on rare tubes?