I would Guess Marshall or Dumble or both did not like the comparison... if you notice, PRSh never really named either in public for that reason..
Actually, they'd have no legal reason to object to the comparison, or the name, since they have no amps called MDT. Just as Cadillac ads compare their products to, say, BMWs and other carmakers, that's fair game. Paul would likely be well within his legal rights to say, "This amp can sound similar to a Marshall, a Dumble or a Tweed."
So I doubt PRS changed the name for that reason, though if someone objected I could imagine that he might be a nice guy and simply go along to get along.
If another amp was already on the market, and called an MDT, that might be an issue - but might not at all, depending on circumstances, prior filing status, etc.
Trademark law is designed to protect registered marks, to prevent marketplace confusion, to keep trade dress unique, and in the case of state law, to protect common law marks, or single state marks, etc.
Since MDT doesn't involve the use of the Marshall, Dumble or Tweed marks (assuming that Tweed is even a registered mark), I kinda doubt this rumor.
But anyone in America is free to write a threatening letter, and though there'd be no real reason to go to the mat on a model name that nebulous, it's always possible that someone tried to make a stink of some kind.