Thanks to everyone for their replies so far.
I have played with adjusting the height and pole pieces of the Mule, it seems very sensitive to height changes compared to other pickups, anyone else experience this?
I have got the Mule sounding better in my guitar (with the adjustments above, and the tone knob of the guitar down to 6 - 7), to a point where I don't dislike it, it's just not the sound I was hoping for. In short, still a little too bright, but also a little underpowered and slightly too mellow/polite. I would like something with a little more output, but not anything near a high output or metal type pickup, and a little more raucous, more weight behind each note.
As stated in my initial post, I have an Abraxas which in my opinion could be described like this, but I would like to try something different, which is why I chose the Mule over another Abraxas in the first place.
I am considering a Black dog or Crawler (as suggested by Telerocker). My concerns are that both may still be too spikey in the high treble (as Alnico 5 magnets), the Crawler may be too high output, and the Black Dog maybe too nasal for my liking due to the pronounced mids.
What are others experience comparing these pickups, or would I be better off sticking with another Abraxas?