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

Based on my experience, the Mk II version is actually fuller and more fleshed out then Mk III. To me Mk III was simply too dry and cold. The differences are subtle and I could easily live with any of them, but if I had to chose, I would go for Mk II.
I recently retubed mine with new 6550's and I'm glad I didn't order my tubes from ARC. The price they charge is simply ridiculous. For $200 you can get a tightly matched octet of Svetlana 6550s (same as ARC uses) from at least two reputable online tube dealers. The 6dj8/6992s are not expensive either and remember that they will last much longer than 2000 hours the 6550s are rated for.

I noticed a significant improvement when I rolled the tubes in my ARC VT100mkII, but it was no walk in the park. Replacing the power tubes was simple enough. I would definitely recommend the Svetlana KT88s over the stock Svetlana 6550s: they will provide you true tube warmth and liquidity. For about $500 more you can buy premium tubes, but you need two matched quads and replacements may be difficult to find. Thetubestore.com will sell you the matched number-coded Svetlana KT88s, so buying replacements is not a problem.
When I tried to roll the four pairs of driver tubes, I blew up one hell of tubes and resisters. Chris at Audio Research told me I should not attempt to replace the "stand alone" pair of Sovtek 6922s located physically towards the amp's front end with any other brand, as the amp cannot be properly biased without them. But I had already found a tube that would perform in that position and not blow stuff up -- the Mullard CV5358. And I had done enough experimenting to know I didn't want the Sovteks, which in that position really limited what the amp could do. Bought the Mullards NOS from Michael Wharton at Brit Audio. While the amp can be biased around the Mullards, and their huge soundstage is a big improvement over the Sovtek's rather lifeless image, they are lush to the point of sounding mushy. Matching them with three pair of relatively clincal/analytic sounding Amperex 6DJ8s (orange globe logo, Holland, steel pins) did the trick.

I love my Mk-III but wish I'd gone for an LS-25 Mk-II and a good solid state amp (Bryston 4B-SST, Gamut, etc.). Yes the $850 retubing charge seems like highway robbery! It's Upscale Audio for me next time around!

Mrwigglewm, that's exactly what happened to me, I tried to replace the driver tubes with JJ Tesla E88CC and blew the resistors and a transistor near the closest tube to the faceplate. These Teslas would not bias in VT-100MKII. I would probably not mess with driver tubes any more, however your describtion of Mullard sound is very tempting. Audio research is installing the 6922 for me. But I will try the KT-88. I talked to Kevin Deal at Upscale Audio and he actually recommended the new Electro-Harmonix KT-88. They are supposed to be even more dependable the Svetlanas.

Gvatchna, I'm glad I read your response. I was going to try the JJ Tesla 6922. Did ARC comment on whether the electrical characteristics of the JJ are that different from the Sovetek? Did you actually adjust the input bias controls? I know how to do this after an intensive learning curve, but I would not recommend this procedure to anyone unless they are used to working on electronics, and, in addition, have a lot of patience and are willing to learn a number of things in the process.