Hi and welcome :D
I second everything Jeff says.
I'm not familiar with SD 59s personally, but on what I've heard, if you've swapped the magnets to IV, then I'd say your reasoning about standard IV Mules is sound.
However, what I can add is that my Mules were originally in a bright sounding Epi LP and they weren't quite "doing it" for me - I'd been expecting a slightly "fatter" tone.
This might be because I hadn't mastered pickup height adjustment to get what I want, but I didn't leave them in there too long to find out because I had a few issues with the Epi anyway. I bought a Tokai Love Rock to put them in and everything turned out peachy :D
I've read since, that in a bright LP, maybe the Mules weren't the best for what I wanted. I understand that one of the options to tame some of the brightness, fatten it a bit, but retain the vintage vibe in a bright LP is to go for Black Dogs...
It all depends what you want, of course. I suspect your best bet is to call BKP/Tim and discuss your options. I've never done it myself (yet), but from reading on here, he is obviously the man to sort you out!
Here on the forums, we'll all sing the praises of BKPs in general, and help you as much as we can, but before long we'll have suggested every pickup they do as the best solution :lol: