Heavy, well let me try my best to explain this. It seems that I'm one of the few that want my gear to sound like an actual concert, right or wrong I feel that the ultimate musical experience is a concert. That is the origination of music and thus it has trickled down into gear that tries to replicate this sound. Now, when at a concert, say this is a acoustic performance for simplicity's sake; when the artist strums the guitar it has weight. Each strum is not only heard but felt by the audience physically (albeit also emotionally but we'll focus on the physical sense). Even though the strum is not a heavy bass hit that reverberates at your core, there is still a distinct sense of weight that it carries that your body feels. This same strum, when reproduced in most audio systems does not carry weight, it is not heavy. You hear all of the details, the pick rubbing each twist of the guitar string but there is lack of presence because the strum cannot be felt by the listener. So when I say that these headphones are heavy I mean to say that unlike ohhh the AKG701, every sound is not only heard but also felt; the sounds carry weight. I'd like to also separate out dark and warm. I feel that these are a bit different. The "heavyness" is a physical sense where dark and warm are not so much felt as heard they are adjectives where heavyness could almost be considered a verb in this context.
I used to build speakers for a hifi shop (we used old Western, Altec, Jensen drivers and horns in combination) and before we actually put them in a cabinet we would play them through a EQ board. Taking each driver and EQ'ing it so that its frequency response was dead flat. That way we could compare each speaker's innate sound to the other without also listening to its inherent frequency response. Even though they all had the exact same frequency response, when played though the EQ, they still sounded different based on the materials used and design implementation. To this effect some sounded dark almost as if the same sounds were just a shade darker when compared to the other driver. The SM3 does this by accentuating the lower frequencies more so than the higher ones but the drivers are also just a bit darker sounding right off the bat. Now take the AKG701 again for comparison. This is a very bright headphone due, in part, to its accentuation of the very high frequencies but also because drivers used are made of very taught materials allowing them less reverberation hence a very fast and bright sound. Most paper drivers hit the sweet spot. I'm not sure how balanced armatures work so I cannot comment on these. As for warm, well, go back to the 60s and listen to Eico or Accrosound gear. That old tube gear, perhaps most iconically a EL84 based amplifier with old iron transformers, sounds very warm. I'm not sure what makes it this way, call it the lack of EPA restrictions when building their gear so all windings were slow drawn and carefully allowed to anneal, but they are just warmer sounding. New equipment like all this 100 watt KT88 gear is lean and harsh with a very quick sound that focuses on delivering power over delivering rich sound.
It is very difficult to describe these things without just sitting down and listening to a couple examples side by side. The most accurate description I can possibly give is this. Grab an old tube amp or radio for that matter and plug it in. Compare that sound to the gear you currently have. Odds are the older gear will sound heavy, warm, and because of these attributes a bit slower. There is a euphoric sense about this old gear and the Aud-5x does a great job at replicating this to some extent. After all, do you want to sit down and listen to your music like binary code going in and out of your ears or would you like to soak in its presentation and be swaddled by the emotional responses it elicits? IMHO music shouldn't be a hearing test, it should be something much more emotionally immersive than that. The Aud-5x representing the latter in body and soul.
I got a little carried away there but let me know if this helps lol.
There have been some great appreciation posts in this thread already!