When to replace 6550c on ARC VT 100 III

When should I replace the 6550c on ARC VT-100 III?
The unit is a little over 2yr old and is used often.

Will I hear a change in the quality of sound that indicates tube wear, or do you just wait for them to fail?

Will Fresh tubes sound better even though I don't detected any trouble?
New power tubes will typically "freshen" up a sound which has lost some of it luster due to tubes of advanced age. But, whether your tubes need replacement depends on the hours. Tubes in you amp should probably last between 1500 and 3000 hours. If your past 3000 hours you probably should get new ones. One thing you could do, if you know how, is to rebias your tubes. That will usually let you know how worn your tubes are (badly worn tubes won't bias up properly) and if they don't need replacement, the rebiasing will improve the sound. Ideally, you should learn to do this yourself and do it every couple of months to get the best out of your amp.
I have seen two methods of rebiasing. One seems time consuming, using two meters, to match both channels I guess. The other is very quick and requires just one meter. Is it worth the time to do the two meter method?
You might want to check out this link from the Tube Store: http://mycollins.net/audio/artube1.html One of their customers sent them his procedure for biasing a VT100. One of the down sides to tubes is they do tend to fade away slowly so the performance decreases but you don't really notice it. One thing you don't want to do is take a tube to failure, that can be messy!
Be aware that the tube biasing procedure in the "mycollins.net" link above is for the VT100 MKII. The MKIII has a significantly simpler procedure requiring only 1 meter, and a few minutes of time.
You should teach yourself how to bias the 6550's. Then, if you check your bias on the present tubes, and it is considerably below spec, your tubes are getting weak. Bass response will become flabbier and weaker as they age, and they will lose some of the 'life' in the sound.

CAVEAT: You can electrocute yourself, or blow components on the board, board traces, and even parts of the board itself if your meter probes, or your hand, makes contact with the circuit board traces, or the leads of a resistor, etc.

There are two large resistors at the top of each large circuit board on each side (channel) of the amp. You really need to buy 'loop' ends to slip on the end of the pointed probes that most meters come with. You can buy cheap ones at Radio Shack. If you have a steady hand, and are careful, you clip the red loop to the lead on one side of the resistor, and the black loop to the lead on the other side of the resistor. You set your meter to mA and look at the reading. To change the bias, there are small blue Bourne pots have screwdriver slots, which you need a tiny flat blade screwdriver to adjust. Or, there is a tool that comes with each VT100 for this purpose.

It sounds complicated, but it is really a snap once you get the hang of it. You can practice on connecting the loops and getting the screwdriver in position, with the amp off and unplugged. You'll then get confident enough to turn it on, let it warm up 20-30 min. and adjust away.

The link suggested in an earlier post is good to look at, but it shows how to bias a VT-100 Mk II, which has only one pot per channel, instead of two. Also, these instructions concentrate on the extremely complicated biasing of the driver tubes, which you do not have to do on the Mk III version -- thank Audio Research that they ended that nightmare of a job! Hope this helps.
Thanks to all for the valuable information. I will check the bias and adjust if necessary. I will get back with the results.
Ok, I'm finely getting back with results of rebiasing.
It was long over due by the sound of things. There is such a dramatic improvement in all aspics. The best way for me to describe it in one word is “impact”.