recordings now scratchy and hollow...and I've changed nothing

It's a bit odd. Neither the mic nor the preamp appears to have anything in the way of a bass roll-off switch attached to it, and it doesn't sound at all as though there's anything wrong with the preamp. Listening to your mic tests on some expensive monitors left me with the feeling though that the one you say is faulty sounds about correct. and the ones you like are bass-heavy! What I can't see (considering all of the tests you've done) is why - unless there's a bass roll-off switch somewhere either on the mic (the most likely place) or preamp that Rode forgot to mention? I'm assuming that as the preamp is a class-compliant device, the signals from it are passing through Microsoft's sound controls, so that would be the next place to look to see if anything weird has happened - like a sample rate conversion you weren't aware was happening...

As for dying-out preamps, I can only think of one possibility, considering that it isn't distorted. And that would be that one of the coupling capacitors inside it has dried out significantly, and lost a lot of its capacitance value. This would certainly cause the results to suddenly become bass-light. Generally though when that happens, other things start to happen as a consequence, and anyway, it would be a gradual failure rather than an overnight one. So if you can, then try it with another preamp, although I must say that I'm not too hopeful of a result...