If it's blackhead, you can treat them with metronidazole. It's sold over the counter in pet and also some feed stores in the States (I'm in the UK). You need to dose them 50mg per 1kg (bird weight) morning and evening. It's marketed as a treatment for fish so I think the most common and easiest way of getting hold of it is a brand called "fish zole".
I read recently that blackhead used to be treated with sulphur, that may explain why the sulphur based stuff you gave them showed an improvement. There's also a thread on the go in the turkey forum titled something like "hope this saves a turkey from blackhead", detailing treating blackhead with Cayenne pepper. This didn't cure my birds but it has bought us some time. It's early days for me as I only started them on the metronidazole yesterday but they are showing a large improvement already with some pink pigment returning to their heads (my birds were pretty far along and had all gone very grey in the head - two of them were dark purple).
Good luck with your birds.