Viva, no longer a fan

I can't imagine there is a ton of grey market sales of an esoteric set amplifier manufacturer. 

If he wants to pay for transformer upgrades, them refer him to local Viva dealer and all should be good?  

Refusal of service on Grey Market audio gear regardless if purchased new or used is done to protect those of whom had purchased thru an authorized dealer. It is an attempt to protect brand value plain and simple. If someone knowingly purchases from the Gray Market, then its a risk that you accept when service is needed. On the other hand if purchased used, and the seller did not disclose the he bought from the Gray Market, then sadly you are stuck with no recourse. Good Luck! 

Not servicing used products unless purchased from an authorized dealer is overly restrictive to Viva owners unless they couple it with a liberal trade-in/upgrade policy.  In effect they are limiting the resale value of existing customers' equipment by strongly discouraging sales to third parties.

I understand their policy regarding grey market goods.

Viva is a niche player.  In all probability neither of these policies will hurt them in their intended market.  In the North American market you have to really want a
Viva to buy a Viva.

One of the things you give up when you buy used, is being treated by the manufacturer as if you had purchased it new. There are some manufacturers who, as a courtesy (Roger Sanders, Jeff Rowland, Loyd Walker, Nelson Pass, Ralph Karsten and others) go out of their way to offer their support to the user of their gear without question.  I'm not sure if those courtesies would be offered to me if I were overseas, or if the manufacturers sales were exposed to gray market competition.  

Read literally, the distributor’s note implies that the company will not service any equipment "purchased on the used market from anyone that is not an authorized Viva dealer." I wonder if they really intend to proscribe that broadly? Say you are in the States and want to buy a used piece of gear from a private seller (or even a non-Viva dealer who received it as a trade-in) -gear that was originally sold by an authorized dealer in the U.S. market. No service or support because the (used) purchase was not made directly from an authorized dealer? Even though the product was originally sold by an authorized dealer within the territory?
I do understand the grey market issues- and those are legitimate concerns both to protect the marketing/distribution system and the integrity of the product (especially if there are regional product differences that may involve legal compliance).
I like the ViVa gear I’ve heard. I doubt that the company intends to foster ill-will among existing owners or prospects. I’m not entirely sure of the facts in the OP’s situation--he says the amps are in Manchester (England?) and wants to upgrade and use them Stateside? (Thus the response from the U.S. distributor addressing a voltage change, among other things?)
If the gear had been purchased from an authorized dealer in one country, and the owner (whether or not the original purchaser) wanted to relocate to another part of the world, the customer ought get support. That would seem to have little direct impact on the grey market unless the customer’s objective was to "flip" the gear- but in that case, why bother with the voltage change? Why not just resell in the "foreign" territory?
My bet--the company might revisit this broad statement of policy to ward off grey market issues without alienating or limiting their customer base. If it is the company’s intention that all resales of equipment have to go back through an authorized dealer, that should be made clear. I had experience with this at one point with a car- I was required to sign a contract that said I wouldn’t "flip" it privately but if I wanted to sell it, must resell it to the authorized dealer--this was to prevent premiums for a then "hot" product that had limited allotment. I didn’t have a problem with that and the time frame was limited to a year after purchase. (Even in the case of cars that are built as "world market," there are compliance issues that vary by country, but that’s a separate matter).
I know that sometimes, posts like this are constructive, because they bring attention to a situation that has put a customer in an untenable situation. Sometimes, it is the communication process- frustration leads to annoyance which leads to unintended outcomes for both parties. My experience with big companies- and ViVa as far as I know is a small company (unless they are owned by a large operation) is that the wheels of bureaucracy are frozen and almost impossible to turn. Let’s hope this situation is clarified for all concerned.