When to replace 6550c on ARC VT 100 III

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.