thought some of you might like to have an idea of the difference in my greco. i asked tim for a riff raff with the mids of a mule, or a mule with the bite of the riff raff. he suggested A5/A4 mule set (with A5 bridge) and even potted them in my orginal greco covers.
the magnet is changed, but also its wound a bit less hot. to preserve the mids with the A5 magnet tim did some magic with the wind.
i dont know about the pickup technical side, but i can tell you the difference tonally.
due to tim's intervention, the mids were exactly the same (or very close) compared to A4 (i have a4 mule in another guitar) and A5. the lower bass and upper treble are much stronger in the A5 version - it was necessary to modify my amp's eq.
i wanted the mule with more bite, and thats essentially what i have, the A5 magnet gives that extra bite, but there is a trade off with the extreme frequencies, which are stronger - fixable on the amp.
with an A4 neck (tims suggestion) the middle A5/A4 position is wonderful - really good. A4 mule neck as versatile as ever.
A5 bridge is hotter than A4 definately - I imagine the pickup is actually somewhere between A4 mule, A5 black dog and A5 riff raff.
i would suggest considering an A5 mule for SG and Flying V guitars. Les Paul and Explorer would probably be better with the A4 model. i can see why tim suggests A4 in general - as it would work in SGs and Vs too... but the A5 adds that nice bit of bite in these barky guitar models.
all personal opinion of course ;) essentially the A4 version is more vintage sounding, and the A5 is more modern. both are versatile and sound perfect clean or driven.