Yeah, it sounds like a Martin Dreadnought to me too. I'd try to find a dry sounding room. If you don't have one, hang up some blankets. Try to block reflections from hard surfaces.
It's hard to get a great sound from that mic. I've used it before, and while it seems like it could work for a lot of voices, I would rather use an SM57 for acoustic guitar.
You might try to find an Oktava MC-012 or maybe even more useful, a really nice dynamic mic, like an EV RE20, or a Sennheiser 441. Even if you upgraded your entire studio to awesome ass world professional, those mics will still get used.
I do think the Studio Projects is a killer when it comes to acoustic guitar. It sounds scooped in the mids, w/ a really bright high end.
Doesn't the video of Hurt show him singing and playing into a couple AKG 414's? One might be attainable, but they're pricy. But the guitar and room are more important. Try letting your strings go dead. Johnny Cash on the American albums doesn't really have a classic beggars banquet sort of dead strings sound, but you'd be surprised how much better things sound, especially with a mic that has an exaggerated high end like the S.P.
Anyway,
FWIW, the Johnny Cash song sounds to me like:
Martin Dreadnought + Condensor (or maybe not) + not very live room + 1176 compressor. But I don't really know. Just guesses.
I don't think it matters if it's a condensor or not. But I'd really try to find something very flat in the high end and midrange, and then compress pretty hard.