ARC VT-100MKII to MKIII, upgrade worth the cost?

I would leave it as MkII. I did some extensive comparisons between Mk II and III and actally liked the Mk II better. In my opinion the Mk II version is simply more musical with fuller, more palpable midrange. The Mk III seems dryier, brighter and sort of "mechanical".

I agree with Markxiii. I would add that the upgrade from MarkII to MarkIII is not nearly so dramatic as the upgrade from MarkI to MarkII. The presentation is a bit dryer and the breakin period is fairly long. YMMV.

One thing you do get with the MarkIII is a 12 volt trigger. This allows you to interconnect certain Audio Research preamps with the amp and to use the preamp remote to turn both the preamp and amp on or off.

It's also my understanding that the tubes are supposed to be longer lived and more reliable.

If you do decide to do the upgrade, summer time is the best time of the year turnaround time wise. The guys and gals at the AR plant aren't quite as busy.

Thank you for your feedback. Leonard at ARC told me the sound is fuller with MK III. Do you find it to be true? MK II sounds very good but a little light weight to my taste. Also, they charge a fortune for retubing and financially it almost makes more sense to upgrade., unless I go for tubes from other retailers.
Any thoughts?