tl;dr at the end if you don't want to read my life story

My headphone journey has been very simple. Sennheiser PX 100 (by chance), stuck with Senn as I upgraded: 558s for years, then 58X like two-three years ago? and finally added an O2 SDAC to the mix. Also rocking some Sony SSCS5 speakers for what it's worth.

I like the 58X with oratory1990's eq on them. Love them in fact. I actually did test them against the 6XX and overall preferred the 58X which might be heresy. If I remember correctly the 6XX had a wider soundstage and more detail but it felt lacking in the punch I want. So i've been happy with my purchase for quite some time.

But I'm starting to realize they do tend to get busy in the mids and lose out on detail sometimes, and I really wish they had more subbass extension and presence. I listen across genres though typically end up listening to classical, IDM, Daft Punk and whatever you call Random Access Memories, and Carly Rae Jepsen style pop. I wouldn't call myself a "basshead", I just am a professional bassoonist and like to actually hear the true bass instruments in orchestral music properly.

Like, if you don't get at least a small amount of stanky rumble from the contrabassoon in the passage here https://youtu.be/a9TCjM9jhks?t=279 it doesn't do it justice. The 58Xs do the job well enough, my speakers deliver it slightly better. But I want more.

I'm actually demoing the new Hifiman HE400SE because I was curious if I could really tell the difference between dynamic and planar. I like how smooth they are and do notice a slight amount more of detail from them when things get really busy, but in most songs actually feel like they do a worse job than the 58X. (They do pass my stank test but only barely). They do a noticeably better job than the 58X at playing the start of this without sounding like a jumbled mess https://www.youtube.com/watch?v=8KjoPFFh_SE?t=10 which to be fair is so often played as a jumbled mess I'm not even sure if this recording isn't just like that.

My go-to bass test song is Doin' it Right though https://www.youtube.com/watch?v=LL-gyhZVvx0 - bad setups can't even play the lowest note which is just sad (and not doin' it right). Again the planar does fine with this but I love the extra oomph a lot from the 58X and the overall sound of it better.

So I'm torn. I'm not even sure if upgrading at the price point I'm looking at is going to make a noticeable difference at all. I suspect it might.

In summary, my research has landed me on the Sundara or the HarmonicDyne Zeus as my two big contenders. I suspect the latter will be more what I'm looking for and am aesthetically biased towards it being maybe the best looking headphones in my price range (don't judge me). But I'm worried that Sundara might actually meet my bass needs and then I'm possibly giving up a lot to chase that.

Or is there something else in this range that might be a better bet?

I guess the other option would also be to switch pads on the 58X? That'd certainly be a lot cheaper option.

tl;dr I love the 58X but want an open back headphone that has: more soundstage, more detail, more imaging, more subbass. And I don't really want to give up anything to do so, and don't want to spend more than $400.

The one thing I don't really want to do is sidegrade or "just have a different sound to play around with." I'm inclined to try to sell my 58Xs to be able to afford a new purchase, and I would only do that if I get something noticeably just... better than them.