In answer to a couple questions...
1) My PW is 47º, as it's part of the Mizuno MP-32 set.
2) I can hit the 60º wedge out of the sand in a reasonably consistent manner, and it's the first club I've been able to do that with, so I'm pretty happy with it.
3) I don't necessarily have a set amount to spend, so the challenge was more to find the one purchase that would most improve the set. However, I am probably thinking about spending around $100.
Overall, it looks like the favorite would be a hybrid to replace the low irons, a 56º or similar wedge to complement the 60º, or both. Thanks everyone for your advice (and feel free to keep it coming if you have more), I'll have it all in mind as I shop for my next club or two.