There is actually very little to 'Service' in an amp.
If the amp sounds OK and there are no obvious problems, then I would urge you to save your money.
However, if it's crackling/noisy or has an actual fault, then it'll require repair.
Valves are quite sturdy things and unless there's a fault that causes them to fail, leave them alone.
We often see 40/50 year old amps with the original valves still perfectly serviceable.
If it ain't broke, don't fix it....
If you really want me to service your amp, then it's no problem. This usually involves removing the valves (if it has them) and testing them in the valve tester for emission and an estimate of life expectancy. Checking for dry solder joints and poor connections. Checking all fasteners and screws are present and tight. Testing the operation of all controls and features etc... Cleaning dirt from fans and heatsinks.
Safety checks are also carried out. This entails checking the correct type and rating fuses are fitted and the mains lead, mains plug and chassis earth connections are sound.
Please bear in mind, due to the nature of electronic equipment and the life that guitar amps lead, I could service your amp today and it could quite easily fail tomorrow. I can't predict what would die next when you throw it in the back of a cold, damp van and go off on a 100 gig tour!