Thanks- The necks are thin front-to-back but the 5'ers are significantly wider that the 4 I had, and the SR506 is wider, still.
From what I've read, the SRX's have wider string spacing and the necks are a little beefier than the SR's. All stable in my experience.
The alnico humbuckers, while aggressive and seem wider range with more lows avail than the ceramics, have a little bit smoother sound than single-coil or split coil pickups can, probably because of the wider aperture (sensing area). Nickel plated steel stings get me more growl and definition than others (some more, some less), and the rig they're coupled to can make a huge diff in the tone avail.
I wasn't able to try any of these before I bought, either, and the SR506 was my first, so at that time I didn't know how to assess what I was looking at, altho the seller played it. The SR506 has split coil pickups, more smooth-sounding like you mentioned (could be the pre),and I was going to replace it with an Aguilar OBP 3PPK, but hat got put into a Hohner B-BassVI that had a bad pre- sounds great, now.
Haven't had or played a Jazz, so can't make a comparison.
Even tho the SRX pre is a 2-band, I still get the mids by cutting the bass and treble some and boosting the volume.
As always, best to 'try before you buy', but that's not always an option...
Hope this helps in your choice.