I've only just auditioned the HD800S, breifly, but I've had both the HD800 and the PM-1 since December of 2013. From what I've read, the HD800S is just as finicky regarding your choice of DAC and amp, but with a little more bass and less treble harshness.
The HD800 is a crybaby chameleon that changes color and mood dramatically as you move it from system to system, where the PM-1 seems to eat whatever you feed it without complaining.
Looking back on my journey thus far, I went the way of a lot of newcomers, craving detail and neutrality above all else. The HD800 and, no doubt, the HD800S, is a fault-detecting microscope that will uncover anything and everything that's wrong with what's upstream, and the better your gear, the more you will hear problems with the mastering of many of your favorite recordings.
It's so unforgiving compared to the PM-1, that for me, the HD800 remains an instrument I use primarily for comparing upstream components, where the PM-1 is, hands down, my favorite headphone for simply enjoying music.
If you've heard the HD800 and want something with the signature of the HD800S, but with more bass and with only slightly less resolution (a good thing?), I consider the
Beyerdynamics DT880 600-Ohm, to be a "poor man's HD800 with bass," when equipped with the
Beyer factory pleather pads (providing a better seal than velour pads), as long as you drive them with lots of power (i.e. Oppo HA-1 or anything else that can push 600-Ohm drivers well) and, preferably, with balanced output (which requires a recabling mod that can be had very reasonably by sending them to
BTG-Audio, for example.) Not including the cost of an amp, I'm talking about an HD800 competitor (better in some ways, not in others, but very similar, overall) that only costs about $350, equipped as I've described. They have a lot of dynamic "slam" - in the direction of what I've heard about the Focal Utopia, in that regard, but they are also very resolving, thanks to the "taughtness" of their 600-Ohm drivers - even down into the bass frequencies (a very pleasurable thing) with extension that goes well below the HD800 or HD800s, with absolutely no sloppiness - again, dependent on the power you provide. They are much less fatiguing over long hours of play than the HD800, even on my best rig for the HD800 (Metrum Acoustics Octave MKII NOS DAC + the zero-feedback
Metrum Acoustics Aurix amp).
But when it comes to listening without fatigue, with a signature that suits nearly every track in my library, on just about any combination of DAC or amp, nothing beats the Oppo PM-1. Just plug it in and enjoy. I spent the better part of 13 months looking for a DAC/amp combo that would make my HD800 tolerable. The PM-1, on the other hand, just gives instant gratification.
I'm rambling, but lastly, let me add that the HD800/HD800S don't play as well with oversampling DACs (especially ESS) and/or multi-stage, solid state amps that use a lot of negative feedback. They are much happier with NOS DACs and zero-feedback, or at least low-feedback amps.)