I can confirm that the Prestige preamp (Ibanez PWC-3) is very meh, having had one in my SR4005. It's a 3-volt system (not very powerful and you have to change the batteries often -- fortunately it's just 2 AAs, which are cheap), the Q control isn't anywhere near as useful as a mid-sweep would have been (on mine it stayed in the most narrow setting with a slight boost), and while I hope it's just a freak accident, the treble control stopped working after 2 years (no, it wasn't the pot, I checked

).
On the plus side, there's an active/passive switch, the bass still sounded very nice in passive mode, and I got that preamp replaced with an Audere 3ZB, which was a marked improvement in all areas (except that I don't have a passive mode anymore -- but since the Audere warns me a long time in advance that the battery is dying, it's a non-issue).
As far as the non-prestige SRs are concerned, the new preamps have mid-selector switches, but they're also a completely different circuits: old ones had a Bartolini Mk1 preamp, and the new ones are the Ibanez-made EQB-IIIS circuit. No idea how much of a difference in sound that makes.
Still, they're killer basses, and the SR506 is probably the best bang-for-buck 6-string you'll find, ever.