and a set of planar magnetic cans
Yeah, so I just got a pair of those. I ended up going with the HiFiMAN SUNDARA, since it's widely touted as the best set of headphones under $500:

It definitely has a slightly more neutral sound than my Grados, but the most notable aspect is the bass extension. Bass isn't particularly boomy, but it's very deep and tight compared to my other headphones. My Focals definitely pump out more bass, but the SUNDARA makes for a very nice sweet spot for instances where the Focals are a bit much. As far as I'm aware this is an aspect of the planar magnetic drivers, but I'm also impressed at how the SUNDARAs can manage to play both deep bass and treble with such clarity simultaneously. If I did that with the Focals the treble would disappear and with the Grados the bass tends to get lost.
The treble is slightly emphasized, I'd say, though not as much as the Grados. Live performances and acoustic tracks lose a lot of the brightness and sparkle that the Grados provide. It's still notably brighter than the fairly even, balanced highs I get from the Focals, making for a nice middle ground between the two.
Vocals felt a bit more forward on the SUNDARAs compared to the Grados, particularly on the acoustic tracks I tested. On Joan Baez's Diamons & Rust, I found myself noticing the intricacies in her vocal performance more than I did with the Grados, which seemed to slightly emphasize the guitar.
Build quality feels solid, with a mostly-metal construction. The earpads are soft, fairly breatheable, and comfy. The headphones didn't feel practically weightless like the Grados, but they still weigh in at a good 100g lighter than the Focals. Clamp force is reasonable, feeling similar to the Focals in that regard. The up/down adjustments on the earcups feels a bit stiff, but you can be reasonably certain that they won't move when you lock them into position. The cables are detachable, as one would expect from $350 headphones, and the TRS connectors lock in with a satisfying click. I've read reports that the cable isn't particularly high quality and is prone to breaking. I'm not sure if that's still an issue, but since they use standard 3.5mm TRS jacks for the input it'll be easy to find a replacement.
The biggest downside to these is that they really need an amp. From my iPod, I can drive my Grados like they were a pair of earbuds. It struggles a little bit with the Focals, but at full volume it's still right on the cusp of being uncomfortably loud. With these, maxing out the volume doesn't even bring them to a comfortable listening level, and overall tone felt a bit flat compared to when I drove them with an amp. My JDS Element II can drive my Focals way harder than one could comfortably listen to them on its low gain setting, but with the SUNDARA I had to switch to high gain and crank it to the twelve o'clock position to get it to my normal listening level. That's really not a slight against these headphones, it's just the reality of planar magnetic headphones. To add another scoop of hilarity, these are actually considered
easy to drive compared to other planar magnetic drivers.
So, yeah, I'm not going to be able to just lean back in my recliner with my iPod in hand and just chill out like I can with my other headphones, but considering I do most of my listening at my desk anyway it's really not an issue.
As far as specific recommendations go, this is kind of how I rate my three main pairs, in no particular order:
Grado SR80e (US$100): An absolute treat for live performances, acoustic, folk, and rock, and an incredible amount of value for the money. Incredibly easy to drive, fairly low-key, and delightfully bright and detailed.
HiFiMAN SUNDARA (US$350): A good all-rounder. Thanks to it having reasonably bass extension while still having an overall bright sound, I find that it sounds particularly great with modern rock, metal, and IDM. For more bass-heavy tracks, the sub-bass was surprisingly noticeable. It definitely leans bright, though not as much as the Grados. An amp should be considered mandatory with these headphones.
Drop x Focal Elex (US$700): Great for anything with deep, rumbling bass. EDM, rap/hip-hop, dubstep (UK dubstep, anyway--US dubstep works best with brighter cans IMO), DnB all sound sick on these, and stuff like trap really comes to life. While an amp isn't needed for these, their impedance is high enough that I'd recommend getting one.
And the sole entry on the "meh" list:
Audio-Technica ATH-M50x (US$too much): While I wouldn't call these bad, I can comfortably say that I feel like they're overpriced. They aren't really flat enough to be useful for monitoring, with a noticeable bass bump, and inconsistent mids, and I find them to be a bit muddy for critical listening. They're okay for what I occasionally use them for (mic monitoring) but they sound like something that should cost $50, but these sell for triple digits. I would look up the price, but recalling how much I paid for these would probably upset me on a primal level.