It's pretty easy for you to change the tubes. Make sure the power is off !! take to top off (just 4 hex bolts) and the tubes are simply pull out and push in the fresh tube. Takes just about 2 minutes in total. (make sure you don't get the tubes mixed up...they aren't all the same types)
I would not buy anything but Nagra tube sets. They rigorously test and match their tubes. I've tried others, and always go back to the Nagra factory tubes.
You might consider a Nagra MPS (multi power supply) to power both your Jazz and CDP. Should be a nice upgrade down the road. Also a Nagra VFS (vibration free system) is another very nice upgrade.
Congratulations !!