Rich, … I think Jim is giving you good advice. I would like to reiterate what I posted. Call ARC and talk to Evan. Ask him what the best course of action is. If Jim's assessment is correct, this is not a do-it-your-self'er.
I can't make the economic call for you about whether you should movie up to the VS-115, but if your VT 100 MK I is long in the tooth and needs a retube (and possibly other work), I would call Evan and ask him if this is something you should attempt.
If Evan tells you that retubing the VT 100 is like doing brain surgery on yourself, then an old Yoga Bera'ism comes to mind, "when you come to a fork in the road, take it." Either send the VT 100 back to ARC (or an ARC authorized service station) or swap it for something else.
Been there and done it with many ARC products, including the VS 100, the VS-115, the Ref 150 SE and so forth. ARC customer service is first rate.
BIF