I got the chance to audition the MM-500 versus my LCD-3 at my local headphone shop for an hour and a half, using the single-ended out of a Hifiman EF400.
The first thing I noticed was that the MM-500 is very fast with an intimate but open sound. Everything sounds like it is happening in or right against your head with images being very large. Though the separation is good, it is offset tremendously by the fact that the images are packed so closely together; nothing really has the space to breathe. The second thing I noticed was that it can be a bit shouty with vocals. Some of this has to do with the mixing/mastering of the music while the rest of it has to do with a peak somewhere in the lower treble and upper mids. For context, I find the ZMF Eikon to have a very similar peak and fatiguing nature, but I don't find the HD800S or Focal Clear to be fatiguing; chock that up to ear shape and hearing loss I guess. I am not a mixing/mastering engineer, just a music enjoyer with an over-eager headphone budget, but I do think that this would be a very strong choice for a studio environment given its strengths.
In direct comparison, my LCD-3 sounded overly smooth, dark and warm (basically like an LCD-3 should sound) it's not that the LCD-3 lacks details, rather it is a much more "lay back and relax" sound signature that also includes details. Between the two, the LCD-3 has a worse timbre but a much richer midrange; it's kind of a hard truth on one hand and a seductive lie on the other. The bass on the LCD-3 is more prominent and forward but the MM-500 feels tighter and quicker down low. I think that if you already love the LCD-3 and old house sound of Audeze (like I do) you should not sell your old Audezes to fund an MM-500 or LCD-5. This new direction is focused on resolution as opposed to musicality. That's not to say that the MM-500 is not musical, it just isn't willing to lie and butter up your music for increased enjoyment.
The LCD-3 also staged further away than the MM-500 but seemed to have slightly smaller image size. Coupled with the dense black background of LCD-3, it made everything stand out more in the soundstage when compared to the MM-500. I think some of this has to do with the larger double-sided magnets of the LCD-3 versus the smaller single-sided magnets of the MM-500. I suspect that this difference coupled with the more aggressively angled earpads made up the majority of the differences between the two in presentation.
As I was listening to the MM-500 I was trying to consider which of the headphones I've owned that it most compares to; I've settled on the ZMF Eikon and Mr Speakers Ether Flow 1.1 (EF1.1) as the best comparisons.
The MM-500 sounds to me like what I wanted the EF1.1 to sound like: fast, neutral, accurate, and lively. I often found that though I loved the neutral and accurate tuning of the EF1.1 it was over-dampened to the point of sounding clinical; it was a headphone that to me came across as stern and exacting with a scientific delivery of everything that I listened to. Whereas the EF1.1 gave a laboratory-perfect representation of music, the MM-500 has more fun and retains a neutral and accurate presentation.
The thing the Eikons and MM-500 share (IMO) is timbre and their ability to give me ear fatigue startlingly quickly. The Eikon is much slower and has more body to its sound which helps its naturalness tremendously (the Eikon is the only headphone that to me sounds like an actual voice as opposed to the recording of a voice). The Eikon's naturalness is not matched by the MM-500 but its timbre is. I found that the MM-500 consistently sounded right with vocals and natural instruments in a way that compared favourably to the Eikon and exceeded the LCD-3 and Focal Clear. Where I find the Eikon to be too slow and a bit flattering overall, I find the MM-500 to be much faster and timbre correct but only as natural sounding as a planar can get (not very). For me, I will trade the unnatural speed of a planar to the more natural slowness of the Eikon in a heartbeat.
Would I buy the MM-500? Yes, I like the presentation and the neutral sound signature tremendously. It does everything well in its limited soundstage and sacrifices (IMO) very littles to do it. Would I trade my ZMF Eikon, Focal Clear OG or LCD-3 for it? No, all of those headphones equal it and beat it in one place or another or just flat out do things differently enough that they are all different and worth having (to me).
