I decided to listen to a load of clips in the Players section to see if that helped to clarify things. I started on the assumption that I was better off leaving the Holydiver in my Jackson and keeping that as my more Metal focused guitar.
Black Dog - Great Rock pickup that can easily do Metal and seems far more powerful than it actually is. Certainly seemed aggressive, thick and articulate but that throaty aggressiveness concerns me as it makes me think of my Nailbomb experience, besides which, do I really want something like that when I have the Holydiver in the Jackson?
Emerald - Sweet tone here. Seemed a bright pickup but with a real Rock edge to it. Most of the clips were of Les Pauls so as my PRS seems a bit brighter than a LP, I concluded that it would be a bit brighter than I was looking for.
Abraxas - The Alnico IV magnet and the description on the website made me think this was a bit more tame than it appears to be. It's certainly smooth and seems quite balanced. A sweet Blues tone should be very achievable but I also found some quite convincing Metal clips. OK, so it didn't have the aggressive edge of the Black Dogs but but if you like your Metal smooth, warm and organic, this can certainly do it. Clearly a very versatile pickup so I can only assume that most people make the assumptions I did as it doesn't seem to be as well represented as it seems to deserve.
Crawler - As most people had either not mentioned the Crawler or said the Abraxas would be better I nearly didn't bother with this one but I found some Rock/Metal clips done on a Les Paul and I'm pleased I took the trouble. This is a really good pickup. In some ways I can see similarities with the Abraxas yet I admit that it is tonally different. It has the versatility, warmth, smoothness etc. of the Abraxas and seems to cover everything from Blues to Metal like the Abraxas. I got the impression that the AV magnet gave it a bit more of an aggressive edge, though I hesitate to use the word 'aggressive' as it seems wrong for something so warm in tone. It was thicker and darker than the Abraxas yet clearly from the same family.
So, have I come to any conclusions? No, not really :( I'd still appreciate further comments from those with direct experience of these pups, especially in guitars like my PRS but I am now certain that I want either an Abraxas or Crawler set.