Poplar is soft like Basswood, and shaves off some top end sparkle and attack. (vs Swamp Ash or Alder anyway) So I imagine you're talking about the high upper mids, and it sounds harsh and whiney, but not the really high treble "acoustic piezo" type of plinkiness. So a Custom Custom type pickup, that also shaves off the extreme high end, for a smoother attack and upper treble would probably be good, but not truly warm up that Poplar because it's still got a fat upper midrange bell curve. Ash, Maple, yes, but Poplar maybe the frequencies aren't really compatible. Or if you do try the CC, stick with 500k pots to leave some of the attack in there for the sake of note clarity, which Poplar can lack.
The Dimarzio line has trouble with high end articulation, too, so the warmth of the VPAF series comes more from a general smoothing of the highs that Dimarzio's tend to have, vs. Duncan's signature "punch in the face" attack. And Airbuckering does that to an extent, too. A Dimarzio might be okay, but I'd stay with the Tone Zone over the Air Zone, because it has more punch in the lower mids. I'd also consider the Super 3, which compresses and mutes out the upper mids a little.
From Duncan, I'd try the PATB Distortion, even though it still has great highs. It has more compressed upper mids too. It's so hot, that if you did put it with 250k pots, you'd taper off the super highs, AND leave the smooth lower mids intact. So if it were too bright with 500k, you have an ace in the hole. And the Custom Custom with 500k pots should be good, too, but if your problem is really in the upper mids, you might even consider the C5 with 250k pots.