I've had an S2 Vela for several months, and just got an SE Starla.
I'm also constitutionally a Gretsch/Tele/P90 sorta whacker. In my book, P90s and Dynasonics tie for Best Pickup in the Known Universe, and if I had to claim one guitar as my voice, it's a Jet with Dynas. So we seem to come from similar perspectives. I thought I'd sworn off Gibby-sized humbuckers years ago - or at least demoted them to roles as character actors, horses for some but not all courses.
All that notwithstanding, I haven't found any PRS pickups too dark for my ear. Caveat: I haven't played all PRS pickups. I know the 58/15 S; several generations of the 85/15 S; current Core 85/15LT; DS-01; DS-02; at least 2 iterations of the Santana SE pickups; whatever came in an SE Singlecut in 2010; the SE Paul's Guitar TCI S; and Core 57/08. Of that bunch, only the 245s might be considered darkish, but they're in LP-like guitars, so that's the brief.)
And all that digression aside, you wonder how an S2 Starla, S2 Vela, and/or S2 Studio might slot into your Tele/Gretsch/P90 Paul universe.
I haven't personally played a Studio - but as a 1-1-2, I would expect the Studio to cover all the Strat/SuperStrat/screamin'bucker lead bases (which is what I hear in Bryan Ewald's demo at the PRS site). And I have plenty of 1-1-2 (and 2-1-2) experience. I probably prefer the single-coilish tones of the narrowfields to all but the fattest Strat pickups I've heard - and to most any split from a full 'bucker.
I think the Studio would fit into your stable without much duplication of your other guitars. As Ewald says, the narrowcoils' humbucker voice is more like a mini-humbucker or P90 than a typical Gibbobucker. Given all that, I admit the Studio's wide versatility, and its utility as a jack-of-all chameleon. But to me it doesn't have a terribly distinctive, unique voice. That is, all its tones are comparable (but not quite dead-on) to the Usual Suspects: Strat, Les Paul/Gibson.
The Vela and Starla are very much their own animals. (Note: my brief Starla experience is with the SE, but I believe it has the same pickups as the S2.) While they're among the least traditionally-PRS guitars in the line, they're also arguably most unlike any non-PRS guitars as well. Their split tones are eminently usable (though you might want to back down the tone knob to take off the crispy glass and get fuller body) - but there's nothing muddy, dark, or dull about their humbucker tones either. For personality, in the context of your collection, they might sonically resemble a very hot Tele - or a thin mahogany P90 slab. (If your Paul w/P90s is a Special, there's some overlap.) That is, the guitars could get into those territories, and serve those purposes - but that would only be part of the range of either one.
You don't say what Gretsch(es) you have, so I can't compare directly, but I hear the native tone of both guitars as somewhere between what you'd get at a halfway point between the stock Filter'Tron HS pickups and Dynasonics. They don't sound exactly like either, and if I hadn't heard the DS pickups described as maybe-kinda-Gretschlike, I don't know if I would have heard that in them.
Clarity through all frequencies is excellent on both guitars; both tend toward a bright signature. The tone controls are effective in moderating that when needed, but I don't hear dark mud anywhere in their native tonesets. Both guitars have a kind of rude snarl and bite which is pretty compelling. Cheerfully nasty. Full enough, but with a throaty focus. And while you could no doubt dial it in, I don't think of either one as fundamentally lush or velvety. I suppose they could ride in the Twang Rodeo, and I get why they're sometimes characterized there, but it's a different twang than from a Gretsch. Gretschs have kerrang; the Vela and Starla have kwank.
I think if you're looking for conventional, well-recognized "iconic" tones that your current stable might lack, it's probably the Studio for it's Stratastical and bridge'bucker range. If you want a more sonically distinctive ride, it would be one of the others. I don't want to leave the impression that the Vela and Starla are sonically identical - each is its own thing - but they're definitely related.
At least from my impressions so far, the Vela is a bit richer and expensive-sounding, and I love the feel. Mine is the semi-hollow, and it definitely feels more luxurious than the Starla. The Starla is a barely dressed-up street fighter which doesn't know if it wants to take a date to the prom - or crash it and trash the decorations.
But. The SE Starla is lots less expensive than the others - a cheap date (especially used) - and starting there might be an economical way to get started answering your own questions. If you love it, done. If you like it but are curious about big sister Vela, you know what to do. If you hate it, you'd probably hate Vela, so it's off to the Studio.