Definitely not going to use fast tracks or choppers. I want the single coils to be much lower output than the S1. The question really is whether the X2N will be a nice single coil tone when split.
It's funny, I spent years trying to do the exact reverse.
If you really want to have an audible boost when you hit the humbucker, then pretty much anything will come across as louder than vintage-output singlecoils. So, just choose a medium-hot or better bridge pickup that you like, and grab something a little more classic in the neck and middle position - I had a lot of success with a Fender Custom 69 neck and Texas Special middle, though you'd have to grab those separately since I don't believe you can order them directly from Fender outside of a set.
If you want a vintage voiced singlecoil pickup, and you're either not concerned with having a hugely hotter humbucker, or want to use something insanely hot like an X2N, more likely than not your problem with the Ibanez singlecoils is how they're voiced, and not with their output. I've got a set of Suhr ML Standards in my Strat at the moment, and while their resistance is in the 6.5/6.9 range, resistance alone doesn't really tell the full story as they're pretty hot, and can keep up with a medium-output humbucker without much issue. They also sound awesome; they're great, very "classic" sounding singlecoils, just with enough output to drive an amp more than a typical Fender singlecoil. I found them hot enough to hang with a Dimarzio AT-1, so if you paired them with an Evo or something around that line (or just kept them a bit further off the strings) you'd probaby be fine.
But, short version is, ask yourself if you want lower output singlecoils than the S1 because you don't like the output, or because you don't like the tone. There are a number of reasonably hot, vintage-y sounding singlecoils out there that CAN balance a lot closer with humbuckers, if you decide you don't want a hugely audible gain boost when you flip to the bridge.