The pickups have pretty hight output but it is a very quiet guitar, noisewise (I think this is due to the pickup selection switch being closer to where all the wiring is). I have lots of single coil Fenders and humbucking Gibson's and the P90's are, even though single coil, more like humbuckers in their sound and output.
The P90's are unlike anything else I have. I can't be near the amp with my casino when I go 3 deep with the dirt but this is also only playing at bedroom volumes (AC15 not turned up more than 1/3 of the way up). I tend to have some dirt in my amp, an OD, then sometimes add a fuzz or distortion to that.
So I can compare these, I've been thinking about a 339 for a while, but as of yet, only have experience with the one semi. I have a Casino, another full hollowbody Gretsch electromatic 5120, and semi hollow Gibson ES-137.