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I don’t have any polycarb boards, but I do have some experience with the high profile variant of the Planck aluminum case. The stock case is a ping machine with heavy tactile switches in it. My initial setup:
However, per @donpark’s (100% accurate) point, aluminum cases are not inherently high-pitched. I think a lot of that comes down to overall mass of the case and mounting style. In my experience, heavy (1.5 kg and up) gasket-mounted aluminum boards can sound wonderful, even without a ton of foam and other dampening materials.
Eventually, to make the Planck sound better:
- I poured a custom silicon dampener for my Planck case (follow this tutorial by Serendipity).
- Used painter’s tape as a sort of force-break mod for all of the mounting points and any points where the plate seemed to be making contact with the case around the edges.
- Switched from the “secure” mounting method to the less secure option.
- Bought plate foam to go between the plate and the PCB.
- Swapped out the Zilents for some less crunchy, less ping-y switches.
- Changed to MT3 keycaps for a deeper sound.
After all that, the Planck sounded… better, but not perfect. I think the steel plate is partially to blame, so if you have the budget, I’d seriously consider going with the POM plate that’s now available (it wasn’t when I bought my Planck).
Despite the initial case sound, it’s still a fun board to experiment with and it was kind of a fun project to experiment with all of the improvements I made to it. The aluminum case feels well-made and the (green) was nicely and evenly anodized, so no complaints there.
If all of the above sounds like 
and you just want a keyboard without also acquiring a project, you may get more of the sound you want with the polycarb Planck.