Charles- I am not defending Vlad, but put it into context. I assume the OP is overseas, I may be wrong. As far as I know in the US, Vlad has no authorized repair centers; apart from tubes, in my experience the gear goes back to Lamm Industries in Brooklyn for any repair. It seems there are two alternatives here: one, to see if the dealer can sort the problem (my experience with dealers has varied widely over the years, from ones who I would trust with my life and others who are so horrible I wouldn't go public only because it would probably be defamatory). I have no idea how effective the dealer can be in the circumstance, giving him every benefit of the doubt.
The other alternative is to ship the unit to Vlad after making arrangements to do so, with a rough idea of what expected turnaround time would be. Since Vlad builds the darn things, he can get to the bottom of any issue quickly. The biggest obstacle is shipping and delays associated with that. (Although, FWIW, I got stuff shipped from Allnic in days- Korea to NY. Crazy). If it were me, I'd probably opt to have the unit looked over by Vlad.
As to whether this particular preamp doesn't work well with some power amps, I haven't a clue. It does sound like that was one of the messages given to the OP, either by Lamm or the dealer (i'll have to go back and re-read the various excerpts that were posted).
The point of my earlier post was to address the practical exigencies- Lamm is a small operation without the kind of 'customer service' staff you often find at larger manufacturers. (Vlad's wife handles part of the intake and she is charming and usually pretty good about responding to emails). And, I get why, in the first instance, Lamm would urge the dealer to try to sort the problem, particularly if the dealer is close by- the dealer can go to the customer's house, can try and troubleshoot the unit, rather than figure it out over the phone, etc. For that reason, I don't think Lamm is giving the OP the 'brush off.' That said, I still think the best option is to send the unit back to the factory. If it turns out that there is some fundamental incompatibility in how the unit works with certain kinds of amps, and nothing is wrong with the preamp, that's a different issue. But, to definitely determine whether something is wrong, I'd arrange to have the dealer send it back, as described above. Worst case, presumably, if the OP bought it from that dealer, is that he works something out for a different product, assuming the unit checks OK .