Lee,
To be honest with you, I would never give my money to a dealer that would willingly feed me the hogwash that this Plinius dealer is feeding you just to make a sale. I would not trust this dealer and would strongly recommend you go to somewhere else if you decide to go with Plinius -- even if it means having it shipped.
First of all, Nelson Pass is a legend in the audio community for a reason. He has decades worth of experience building some of the finest amps in existence. The current models of Pass amps, such as the X-250.5 of which you speak, are built like tanks and sound heavenly. I own the X-350.5 and can personally vouch for their sound on Martin Logans. They have great bass, rich tonality and midrange, and sweet, extended highs. The fact that there are at least five or six members on this board who are more than satisfied with these amps driving Martin Logans should clue you in that this dealer is feeding you a line of B.S. in order to sell you something.
I had an Audio Research dealer feed me the same B.S. about Conrad Johnson equipment a couple of years ago and I walked out of his business determined never to enter that store again. I simply will not deal with someone who is going to lie to me to make a sale. It is a shame that dealers can't push the good points of the equipment they sell without bashing the competition.
I have never (yet) heard a Plinius amp but from what I have heard from members of this forum and around the web, they are fabulous amps. You really can't go wrong either way and I fully expect you will be satisfied with your purchase. The best would be if you could afford to buy a Pass and a Plinius on the 'gon and then re-sell the one that satisfied you the least. There is an active market for both brands, so that wouldn't be too tough.
Again, I would strongly encourage you that whichever brand you decide to purchase, that you not reward a dealer that would insult your intelligence in order to make a sale. In my opinion, you cannot trust that individual to treat you fairly.