I have both: a 594 with P-90s, and a 594 with humbuckers. I did a song where I recorded them doing a little solo back-to-back to demonstrate the differences, after I got them.
I’ve posted this before. Here’s the link:
https://soundcloud.com/lschefman/messya-deux-v2
P-90s can have a ton of clarity, or depending on how you set the amp, they can overdrive an amp (or pedal) nicely. They can have more warmth than a humbucker; vintage sounding P-90s like on the 594 are warmer than the P-90s that used to be available on McCartys and CU22 Soapbars. However, there are also bright, crisp P-90s that have more top-end.
A lot depends on the guitar, the pickup and how it’s wound, etc.
The rhythm guitar is a very light crunch done with the 594 Soapbar. Obviously it sounds different from the solo, it’s brighter and cleaner; but I mixed it pretty low.
Both of these solos were done playing through a PRS DG30 amp. I may have used an OCD or Xotic BB set for very low gain, mostly as a boost, I honestly don’t remember. Bridge pickups. The volume knobs on the guitar were set at about 7-8, the tone knobs around 8-9.
P-90s are a hot single coil pickup; they will hum in an environment with lots of EMI/RFI, but you can always turn and find a place to face where the noise is minimized. When both pickups are on, they tend to cancel out each other’s hum.
Yes, I know the playing sucks, don’t remind me.