^ Which DAC and amp are you using with the PM-1; is it only the Mojo, driving the PM-1 directly?
It is not a bright headphone. HE500 is just smoother than them. A bright headphone would be the original HD800, which was quite bright.
I agree with George's comparisons. I have the PM-1 and the HD 800, and spent about 13 months in a love/hate relationship with the HD 800, until I finally gave it the signal it wanted, with an NOS DAC - the Metrum Acoustics Octave MkII (stay away from ESS Sabre DACs, especially) and a zero-feedback amp - the Metrum Acoustics Aurix (stay away from any multi-stage amp, like the Oppo HA-1, that use negative feedback to correct the distortion created by such designs).
I have to mention, however, that the Oppo Sonica DAC, equipped with the latest and greatest from ESS, the ES9038Pro, is incredibly smooth in the high frequencies, while also delivering the most detail of any DAC I've owned (which, admittedly, does not include any DAC costing more than about $1000 street). My current, favorite desktop combo for the ultra-finicky HD 800 is the Sonica DAC + Metrum Acoustics Aurix. Oppo's implementation of the ES9038Pro is a big departure from that sibilant, brittle, edgy treble heard with the HD 800 when fed by other ESS DACs (even with a zero-feedback amp).
With that HD 800 backdrop as a reference to how bad a "bright" headphone can be, I've often written that the PM-1 has none of that finicky, demanding nature of the HD 800 - that it can be plugged into anything and sound great. Apparently, it doesn't like the Mojo, at least, not in combination with the excellent treble sensitivity of your ears and a few tracks in your library. :-/
I tried playing those tracks you referenced with my near-field monitor setup - the brightest rig of any kind in my possession - and didn't hear anything annoying. But hey, I'm getting on in my years, so I do have some fall-off that negates my ability to judge such things. Keep in mind however, that it took me 13 months to find a good rig for the HD 800, because of its treble issues.
In conclusion, I would suggest you try putting an amp of some kind between the Mojo and the PM-1. More power would elevate the energy of the bass and mid regions, relative to the treble - especially with a planar magnetic. It would also "decompress" the audio - making it more dynamic/punchy - allowing you to reduce the overall playback SPL, I suspect. (Looking at your signature again, I'm scratching my head wondering how you can drive the HE500 sufficiently without an amp.)
Mike