Like Mark Ray, I also had a CU22 Soapbar. Also had an original McCarty Soapbar with a rosewood neck. And currently have a 594 Soapbar.
Sooo...you wanna know the difference between a 594 with soapbar pickups and my 594 with humbuckers? Just so happens, I recorded both guitars on one of my little tunes with each guitar swapping leads through my DG30 amp. Might as well listen to the guitars in action, as opposed to me yakking about it.
I've posted this before, several times, but not in this section of the Forum.
First melodic line is taken by the Soapy. Second melodic line halfway in is taken by the Humbucker version. The rhythm part is the Soapy.
All other things in the track, from amp settings to pedal settings to mix settings are the exact same. My playing is less than stellar, but I think I did a nice job writing the tune, so I don't feel too bad about the result. Here's the head-to-head in the context of a more fully instrumented project, as opposed to some solo noodling. I'd suggest using good monitors or good headphones so you can really hear the difference well.
https://soundcloud.com/lschefman/messy-20
My comment is that the resonant frequency of the Soapy is more in the midrange, which gives it a more horn-like tone, and the resonant frequency of the Humbucker is more in the treble range, giving it a different tonal balance. The resonant frequency of the Soapy seems to have more amplitude, and the resonant frequency of the Humbucker less amplitude.
Resonant frequency is the point where the high frequency rolloff begins. Many transducers like pickups have a peak exactly at that frequency. The greater the amplitude of the peak, the greater the resonance.