I honestly don’t know - when I first got the bass it had P/J pickups but the core sound was still there. I think low action and a bit of clank go a long way to give definition to the sound. Normally you hear mustangs with flats (I had flats on mine too for a while) but they are really quite growly with rounds.
Worth mentioning also that this bass is always in drop D tuning, again it seemingly defies logic because that shouldn’t work as well from a scale length and tension perspective but it does.
I have no doubt that the woods involved play a part, particularly in how stable the neck is. But I think it’s a combination of everything, some educated guesses regarding pickup choices (tbh my options were limited by the existing cavities) and a bit of luck have been on my side, for once!!
I’ll do a video to demonstrate the P sound a bit more clearly.