I am with
@JimmyM , if the output tubes are holding bias and the amp sounds decent, leave it alone. Tubes don't typically go bad unless they are being used. I have a 87 skunk SVT and it's still running all the original tubes. I had to replace the capacitors in the power supply because the original owner apparently never used the amp, but all the tubes are fine.
I also agree that if you do need to get new tubes, it's a good idea to pay the small premium for burn-in service on the output tubes. Modern output tubes have a pretty high infant mortality rate. The burn in gives an opportunity for bad tubes to self-destruct and also allows the getters to clear the vacuum of residual gases that are left over from manufacturing. The operating characteristics of tubes that have not been burned-in are more likely to drift. So it's best to do the burn-in before the tubes are matched. Basically the burn in makes the tubes more stable so they remain matched longer. Usually burn in service costs about $2 extra per tube.
Most of the small signal tubes can be replaced without concern, but I would urge a bit of caution if you decide to replace the two 12AU7s in the power amp. These tubes are drivers and control the bias of the output tubes. Here's my recommendations:
1. Replace the driver tubes in pairs.
2. Consider buying a matched pair of driver tubes. This is because one 12AU7 controls the bias for three of the output tubes, and the other 12AU7 controls the bias for the other three output tubes. You don't necessarily need matched triodes, which means the two sections (a & b) of one tube are matched. Instead you need matched sections between the two tubes; I.E. triode 1a is equal to triode 2a and triode 1b is equal to triode 2b.
3. Before pulling the old 12AU7 driver tubes, adjust the bias for minimum output tube current, and then let the amp cool enough so you can pull the old tubes. Install the new 12AU7s and follow the bias procedure in the manual. Here's why: when you install new driver tubes, it's likely they will be stronger. Stronger drivers pull the bias down, so the output tubes conduct harder. In extreme cases you can damage the output tubes or other circuitry. So preset the bias for minimal current in the output tubes.
You can replace any of the 12AX7s or 12AU7 in the preamp without in concern. You can also replace the 12AX7 in the power amp without concern. I wouldn't be a bad idea to have at least one spare 12AX7 and one 12AU7 to trouble shoot these tube positions. If the amp is acting up (turns on and holds bias, but noisy or won't pass signal), simply roll a new tube of the appropriate type through the amp to see if you can eliminate the problem.
