If you're going to go the curb bit route, I love the bit smrobs is suggesting.
I use this one: NP Sweet Iron Loose Cheek Western Curb Bit 5in - Horse.com
on Lacey and it works really well for her. However, the shanks are a little longer than the one smrobs posted, so it's going to be slightly harsher.
Alternatively, you could just get a loose ring snaffle (btw, any bit with shanks, no matter what anyone calls it, is a curb-never a snaffle), or a french-link snaffle which is even a littler gentler than a single jointed snaffle. Either of those two snaffles are about as gentle as you can get, bit-wise, and there's nothing you can't do with a snaffle.
Here are a few right off (to help you in looking, if you want):
French link: http://www.horse.com/item/korsteel-f...bit/SLT900229/
Alternatively, sweet iron encourages salivation and I know a horse that LOVES this bit (also a french-link): http://www.horse.com/item/korsteel-s...bit/SLT900268/
Single jointed snaffle: http://www.horse.com/item/ss-loose-r...n/WBM12%205IN/
A curb bit might have a little bit more finesse but you say that she's very soft mouthed and responsive so she'll probably have plenty of finesse in a snaffle. And, with a snaffle, you can direct rein or neck rein instead of only being able to neck rein in a curb.
Good luck! :)