amham, can you elaborate on your experience with the SE and the HE-500's? I have an SE on the way and have been intrigued by the HE-500 lately. I thought I'd heard someone talk about them being mismatched, but I reckon it's a personal taste matter and would appreciate hearing about yours if you didn't mind.
That's the difference between theory and practice, basically. People have been raving about how the HE-500 is supposed to be really hard to drive, need 2000W to sound good, and blah blah blah... But, it's still a sensitive enough pair of headphones, that despite its low impedance (and therefore inherent mismatch with OTL tube amps, especially less powerful ones), has no damping factor issues because it's an ortho (flat impedance throughout all frequencies, behaves like a resistor across the whole spectrum in a way, unlike typical high impedance dynamic phones like the HD650) and doesn't really care about the high output impedance an OTL tube amp like the MK IV would have (which would be the "problem" here: impedance mismatch).
So, unlike dynamic phones, the worst the HE-500 can suffer is being a little bit starved for current, but for the HE-500 to sound "bad", it would take more than just a less powerful amp, you'd need to severely underpower it (think ipod or iphone). High impedance dynamic headphones like the HD650, on the other hand, sound like Schiit when paired with the wrong type of amp and/or when underpowered (think cheap SS amp with some current but very little voltage amplification).
The HE-500 isn't actually very picky with amps, so long as they get some degree of current amplification. And even the MK IV SE will deliver more than enough current to low impedance cans (despite being made for the exact opposite application) to drive the HE-500 to ear-shattering levels. The worst you
could and probably
won't be missing out on is a tiny bit of bass, that only a full-size speaker could give you, and the OTL detail and 3D sound more than covers for that (and upgraded tubes cover that too).
I've been using the HE-500 with my MK IV SE for almost a year now, and I can at least vouch for the fact that there is tremendous synergy between the two. Balanced-sounding orthos like the HE-500 (Audeze's less so from what I hear) just pair marvelously well with OTL amps in terms of sound, it's like hearing ultra-detail in a velour glove or something. And with the right tubes, I dare you to find anything missing with that pairing!
Hope that helps
A happy MK IV SE user