Y'know, if you're using a Diplomat and stretching it to the point your rim is bottoming out... Maybe you're just exceeding the tension that the Diplomat can handle. The thinner the Mylar, the more prone it is to stretch. Also, different plastic formulations have different resistance to stretching (as you saw with the Ludwig head), so maybe another head with a different film would work for you.
So anyway, some other possible solutions:
[list]
[*] Stick with the Diplomat, but at a lower tension so it doesn't stretch to the max.
[*] Switch to an Ambassador -- same film, but a mil thicker. Not quite as sensitive, but more resistant to stretching.
[*] Try heads from other manufacturers: You've tried the Ludwig snare side. Among others, there's also the Evans Hazy 200 (2 mil), Evans Hazy 300 (3 mil), Evans Orchestral 200 (2 mil), Evans Orchestral 300 (3 mil), Aquarian Classic Clear (3 mil), Aquarian Modern Vintage (3 mil), etc. The Terry Bozzio heads from Attack also seem to have a following, but it's the same Mylar that Remo uses, so I don't think that would solve the problem.
[/list]
My next suggestion, which may not be any better than the next guy's, would be to try the Evans Orchestral 300. It's thicker than the Diplomats you're used to, but that'll help to resist stretching so the head won't bottom out and it'll maintain the tuning you like. And even though it's an Ambassador weight head, I've found it to be pretty responsive.
No matter what you try, good luck and keep us posted!
...and as a footnote, I've got Aquarian American Vintage heads on my '56 Radio King. (Mostly 'cause that's what fits!) They seem pretty darn resistant to stretching -- it took over a week for the head to stretch enough to conform to the snare bed! It's got a warm sound and moderate response -- nothing at all like a Diplomat. I'd actually be inclined to say its response is a little drier than an Ambassador. I like it, though. It works well on this drum.