Personally have not been a real fan of most planars - just too flat sounding for me (including Audeze LCD lineup, Monoprice M1060, Fostex T50/T40 RP, HE-4XX.) I appreciate the technical abilities of most planars, but they just don't have the low end presentation and fullness of sound I get from the Fostex Biodyna dynamic drivers I love or the punchiness and dynamics of the STAX, and those are things I value more than speed/linearity/precision. I do like the Audeze Mobius more than the ones I listed above, but it still is not what I'm exactly looking for in a planar.
I have found a planar though, which seems to be what I was looking for and has made me put aside both the HD58X and TH-X00PH right now... and if you know anything about my preferences, you know the TH-X00PH are/were my favorite headphones (next to the STAX L300 Limiteds) and I do like the HD58X quite a bit. This planar I love is none other than the Modhouse Argon Mk3 - it just really needs the right mix of gear to bring out it's full sound. With a good setup getting its full sound, I could listen to the Argon Mk3 and not care that I have the STAX setup... it's that good to me. You do miss out on the full detail, precision, and immediacy of the highs compared to the STAX L300 Limited, but on its own it's quite good.
The low end is simply PHENOMINAL on the Argon Mk3s (with ZMF Lambskin pads) ... much better extension than on the TH-X00PH, and even slightly better than the STAX L300 Limited with similar technical qualities... nice speed, clarity, and punch. The low end is also very full sounding like a dynamic driver as opposed to a normal planar sound, yet extremely clean, clear, and punchy. It's like a planar, dynamic, and electrostat all combined into a single unique planar headphone... also has a very unique soundscape the music is presented in as well. The TH-X00 PH is very 'one-note' sub-bass/bass in comparison and the LCD series/M1060 are just so flat and dark sounding. I would like to try the ZMF Protein pads on it, but from descriptions of the sound differences feel it would probably be a step back from the Lambskin pads for me. The Argon Mk3 may just be my favorite overall headphone... and it just happens to be a planar driver headphone...