How do you know when an older amp needs service?

I'm using Aragon 8008BB amps and they range from about 8-15 years old. How can I tell if they are performing at their peak? They still sound good and they weigh a ton so shipping them in for a check-up would be difficult and pricey. Is ignorance bliss or should I be looking into having them serviced?
Dave, They seem to suffer from bias drift. If you have two or more and they feel (at the fins) to be at the same operating temp, I would say leave them alone. If not, or if they are very hot in use, time for a tune up. Where to go is another story...
You are suffering from" tube amp envy syndrome". This is a common malady among solid state users who see tube amp aficionados constantly tuning and retubeing their amps. The little darlings require constant attention to function at all, while the crass solid state units just sit there functioning perfectly year after year. BORING! How can you have a meaningful relationship with your amp when it does not require anything from you? Fear not! Just treat them as gardeners do their favorite house plants; talk to them when they are not playing. If it did a wonderful job on the last concerto or rock album let it know; you will both feel better. On a more serious note, my own standard is, if it ain't broke, don't fix it.
Very funny and true.
Hello Stanwal (Spoken like "Hello Newman"),
Just kidding. I know exactly where you're coming from. My house abounds with faux plants that are of such quality that most people have to touch them to find out that they're phony baloneys. Yet, sometimes I find myself envying those plant watering, green thumb types who, even with all that care, never achieve the consistent level of beauty that I enjoy. I just don't get it.
Its simple. the fake plants will never have the bloom the real ones do.

ROFL. Great response. Thanks for putting a smile on my face.