Would an ARC Ref 6 be a good match for a VTL S200?

I mean in terms of input and output impedance.

I'm not sure how one determine these things and I have the opportunity of buying a used Ref 6.

I'd call VTL but I bought the amp as a demo and they always ask you where you bought their products from. If they don't like how you bought it they make you feel like a criminal. Why this is so I have no idea as I thought the resale value of a component in the audio world was a selling point. Obviously these people feel you must buy one of their products and keep it for life. To give them their due their stuff is wonderful but I'd never buy another one of their products due to this attitude.

The rule of thumb is the input impedance of an amp should be at least 10 times the output of a preamp. The AR output is 600 ohms balanced and 300 ohms unbalanced, the input impedance of the VTL is 45k ohms. They should work well together!
I run an ARC Ref 40 balanced into VTL MB450III and it is a great combination. The Ref 6 manual states it wants to see 20k minimum so the VTL should be no issue

The Ref 6 manual is here

As to working with VTL they much prefer that you go through a dealer. I suggest building a relationship with a good VTL dealer such as Brian Berdan at Audio Element in Pasadena. Most manufacturers prefer you work through a dealer and this way you should have no problems having your used products supported. The real party at fault in your case is the dealer who sold you the demo. He or she should be helping you
an ARC Ref 6 is a good match for anything
Another general rule of thumb: tube amps usually have a high input impedance, and thus don’t have impedance matching issues with either SS or tube preamps -- even most preamps with higher output impedance should work OK. Some SS amps have rather low input impedances (10K or less), which can cause problems for some preamps (especially tube preamps).

So the old pattern of matching a tube preamp with SS power (to get the "flavor" of tubes without the heat & maintenance of power tubes) requires some care & attention to ensure a good match (and god help you if your SS amp is DC coupled and your tube pre’s output caps fail). Also, many tube preamps have output capacitors, which roll of bass if the cap isn’t large enough for the amp’s input impedance -- lower amp input impedances require increasingly larger output caps to maintain good bass.

A "low" output impedance for a pre is anything under say 400 ohms. A "high" input impedance for an amp is anything over say 50,000 ohms.

That said, I've heard amazing things about the Ref 6's sound from a buddy who owns a new one (and whose ears I trust implicitly), and now I want one too. 
I've got 30 hours on my Ref 6 so far. I'm listening to Aimee Mann as I write this. Trust me-life is short-just buy one. 
I'd probably demo a VTL preamp first.