Well, the Mule is nailing all things 59 HB. The gold standard PAF tone. Alternatives depending on what you want is the rest of the low output range. They all have something else going for them. Really depends on what kind of sound you want. Mule for 59 is always good though.
Did not see what kind of magnet the sp90 uses, but all three of the more vintage BKP P90s, Half Note, Blue Note, and Nantucket, are very classic sounds. They vary in magnet and wind, and therefore also application though.
The Half Note is super natural and sweet (like insanely so) and loves to play jazz and blues. I also like to take it higher, but it has a very specific character then, a bit of 70s psychedelic. For clean to low gain it is insane.
The Blue Note is a step up. Little more edges, which makes it rock easier. Blues and classic rock no issue and IŽd be surprised if it could not do proper hardrock too.
The Nantucket is perhaps the gold standard all in all P90. From clean to blues to even some kinds of metal, all done with classic flavor. Has more cut, agression, etc than the other two though.
The output also increases in that order. The first two should definetly work with the Mule for example, but the Nantucket might not be optimal if you want the bridge to be higher output than the neck, as it should be equal or slight tad higher I recon, but am not sure. Again, I did not see any specs for the Sp90, but I would not be shocked if the Nantucket is closest as Duncans tend to be a bit hotter and such than BKPs, otherwise I would imagine the Bluenote to be good. However, if I apply this logic to the 59er too you could go with the Abraxas, a beefed up Mule, which still is rather vintage.
So far for this post, I donŽt want to create the uber wall.
