I apologize for missing that you saw two techs. I hate it when people skim posts and make recommendations that have already been covered had they read the whole thing. I try to read everything all the way through before commenting.
Agreed on the pre. Ibanez preamps are meh at very best. I have gone full passive on mine (though with different pickups) and did enjoy it. I know there are some folk on TB that are handy enough with circuits to design a bypass for the pre but i do understand what you mean about it not being possible. I've wanted to do a bypass for a long time but because every single thing goes into the board it make it hard. I would suggest just pull the pre completely out just to try passive.
As far as pickups.... Ibanez does make upgrading a pain due to the unique size of the soapbars they use. I got around this issue by installing jazz pups in the cavities then putting empty soapbar covers over top of them. It worked great and really opens up a world of possibilities. Check out these links for more info on what I'm talking about.
http://www.talkbass.com/threads/the-soundgear-club-part-4.1020843/page-22#post-15752163
http://www.talkbass.com/threads/jazz-pickup-installed-on-bass-with-soapbar.1034074/
Part of the reason I suggest different strings is due to the super low tension. My thinking is that is you use more robust strings its going to force more effort from the truss rods and maybe by the rods being tighter it may create more stability. I have an SR400 from 1993 (first year the SR400 was offered) It has a paper thin itty bitty neck. I used the lightest gauge strings on it of my favorite brand at the time and had frequent neck issues. I had to adjust the neck so often that I stopped putting the truss rod cover back on cuz I'd just be taking it off again within a week. I tried heavier strings and it seems to have stabilized. and it only needs adjustment every 6 months or so.
Honestly I believe that the 90's were a very hit or mis time for Ibanez. The focus on super thin necks is kind of a recipe for disaster as your seeing with this bass. Since then they have started leaving a little more heft on the necks and started using stiffer woods and they have gotten way better in the stability department, unfortunately that doesn't really help you now. You mentioned that it couldn't possibly be the right instrument for someone else but I'd like to mention that some people love high action and that would very possibly sold this issue. But these are just my thoughts. as always YMMV.