The following is mostly complete speculation on my part:
Perhaps you need custom feats in your game to support this issue.
Maybe Toughness needs to give hp equal to the character's HD-type when the feat is taken (though even that is small with these large numbers). I confess this is the way I would run Toughness, as otherwise it is worthless.
Perhaps there needs to be a DR feat-tree that grows larger as more feats are taken so that it does not skew too much at low levels.
Feat #1: DR 1
Feat #2: DR 3
Feat #3: DR 6
Feat #4: DR 12
Feat #5: DR 18
I realize that these numbers are quite large in comparison to extant feats regarding DR, but you seem to have a tough situation.
Or perhaps a lower curve of DR on the feat-tree combined with specialist items?
If chain and plate have inherent DRs of 1 and 2 (there is an SRD option allowing it, IIRC), then you can extrapolate additional DR points for masterwork armor, special materials, and you can also introduce a grand-masterwork concept for yet more cost and DR points.
Then, you would allow the non-magical DR from the feat-tree and the armor and the materials to stack. (The materials may be magical, but they weren't enchanted.)
So, perhaps masterwork armor adds DR 3, so that chain and plate DR goes up to 4 and 5.
So, perhaps, grand-masterwork armor adds DR 6, so that chain and plate DR goes up to 7 and 8.
So, perhaps, adamantine adds 5 (or more), so that chain and plate DR goes up to 12 or 13.
Combined with the feat-tree on the 3rd feat, that's DR 18 or 19. This would seem to somewhat alleviate your problem without resorting to drastic magical effects.
Of course, you make it very expensive. 19th level characters should be able to afford it.
And yes, I realize that 19th level characters cannot suddenly pick up three feats out of nowhere.
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The only other thing I can think of is force-field type spells, activated via contingency-type spells, such as I invented for my last campaign.
Sorcerer's Shield, Basic (Abjuration)
Level: 3
Components: V, S, M
Casting Time: 1 standard action
Range: Touch
Target: Creature touched
Duration: 1 hour/level or until discharged
Spell Resistance: Will negates (harmless)
Saving Throw: Yes (harmless)
Description: This spell creates an invisible force field around the sorcerer. Whenever any object or spell strikes the exterior surface of this field, a ripple of light-distortion radiates outward in a circle. The spell provides +4 AC (armor bonus), DR 5, ER 5, and can provide this resistance for up to 100hp worth of damage, at which point the spell will collapse. The material component is a polished sphere of rock crystal worth at least 50gp.
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Further versions:
6th level: +6 AC, DR/ER 10/10, 150hp, 250gp rock crystal sphere
9th level: +8 AC, DR/ER 15/15, 200hp, 1,000gp diamond sphere