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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
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.