Hey there tharnell, welcome!

I have a Zrock2 as well and my understanding is that the Zrock runs the tube at a lower voltage which increases tube life and allows it to stay on 24/7 with ease.
For amps/pre's and such, you are correct, if you keep track of hours, you can add them up and have a good ballpark as to when the tube is getting tired based on the original tube specs. However, tubes can fail at any time, new or old so beware.

My advice would be since you really should have a backup tube for the Zrock (and any other tube gear really), get another one regardless. If it's truly from 2017 then it probably could use to get refreshed. If it's newer, put your brand new one in and see if it makes a difference. If not, put it back in the box and you'll have a spare for down the road. You may even want to get a couple of different brands and see if one or the other has more synergy with your system if you're so inclined.

Good luck with whatever you choose.