Advice on Repairs for Audio Research VT100 Mk II

You could try reaching out to ARC service department. They have been very responsive and helpful in my experience.

First get more information directly from ARC to make certain that this is not a hard amp to maintain, and that it would not demand more than it could offer in the long run. You bought it, you cannot change that, but you can negotiate with your tech, fix it and then try to sell it minimising your loss. Unless you want to keep it, in that case you have to learn how to. All above are solid answers to your question.

Many thanks to everyone for the very helpful responses.

As to dealing with local tech shop, I also prefer just trusting the tech guy to give me a fair price, but when we're talking EUR 2,300, I want to understand what I'm paying for and why. I don't think that's quibbling or disrespectful. As to the burden if an issue comes up later, the shop will bear this burden for only 6 months, which is the guarantee they're offering. It's better than nothing, but not exactly confidence inspiring.

I didn't realise that the input tube matching was so complex, as described in  Jea48's post. This was really helpful and makes me think that the price isn't so bad after all. 

I also thought about contacting ARC directly, but that feels a bit like going behind the back of the local authorised repair shop. I would also expect ARC to defer to the opinion of the local shop. But maybe they can give me an opinion on the expected cost/benefit going forward if I go ahead with the repair, as petg60 pointed out. 

And yes, this amp already feels like a liability. I love the sound but the amp has needed two repairs within the first 6 months, so it's really put me off the whole tube amp thing. The first repair was replacing two fan resistors, which I did myself and was actually kind of fun, but I didn't enjoy it enough to say, yeah, I'm going to invest the time and energy to learn DIY tube amp repair and replace 20 resistors. Thus, my idea was to get the amp repaired, sell it, and go back to transistors. But if I go ahead with the repairs, I will have invested EUR 4,700 and I'm sure I wouldn't get that much on the second hand market. So either I accept the loss and move on or I use the amp for (hopefully) a few years before the next repairs are needed. If I knew I could get 4 years out of it without another repair, I would choose this option.

Dump it. Buy a ss amp. Not worth the 2k repair charge. 
call it a learning lesson. 

ARC amps are built well and tend to be reliable. Until they're not. I believe most ARC experts and fans will admit that there were some clunkers along the way, in terms of sound, design and reliability. I had a VS110 (which was the lower-tier amp contemporary of the VT110 IIRC) that was remarkably reliable for ten years of constant usage. 

Most-but not all-tube amps are expensive to maintain. My ARC Ref 150SE uses eight KT150's. That means paying $1600 or more every three years more or less to re-tube just the power tubes, closer to $2800 if you opt to buy ARC's "specially screened and matched" tubes.