I received my HD681 (regular, with the red rings) in the mail. The headroom graphs are correct - this headphone suits
my ears perfectly, as does the K271 mkII. I discovered my personal preference after sitting in many audio stores and trying on headphones, finally finding the K271. What I was looking for was subjective flatness - no peaks or troughs in frequency response. A behavior such that if you were to listen to a sine sweep, there would be no sudden ups or downs in loudness. The theory was that good speaker systems always behave this way when I test them, and the presence of any area suddenly too loud to quiet meant when an instrument played there, the audio dropped out or became intrusive. For some examples, the K240, relative to the K271 has a wide peak at 100hz which makes it sound moo-ey to me and the SRH1440, having a similar curve except for a wide peak at 3khz, makes it such that when, for example, a guitar plays a note in this area, it is suddenly painfully loud and honky (8db louder than everything else!)
As such,
the major factor in your decision as to whether you'll like a specific model may be FR graphs. Find the superlux which approaches the headphones you already have which you like the most. Headroom's "Build a headphone graph" tool allowed me to decide on the HD 681, as opposed to the 668B, which would be closer to the K240 in response.
Superlux's website has FR graphs for every model, and makes comparisons to well known headphones in only the frequency response curve.
Review of the HD 681 (0-1 hours burn in
Very clear, clean sound, great extension both ways, no peaks or troughs. Rather sensitive and would work well for portable unamped listening. The cord is very long however. The only way it differs from K271 is the bass does not roll off too early. Not boomy or muddy in the slightest, and appropriate in quantity. The K271, reportedly very bass-light, could use a little more bass if listening for enjoyment versus purely analytically. Depends on your perspective.
It is interesting that many phones considered to be the best, happen to have a FR near this one. Even the T50rp - Thunderpants mod and a few others, are almost the same as the K271/HD681. Some thousand dollar IEM's also have this same FR curve; can't name any off the top of my head but you should find them if you scroll through the 500$+ offerings' FR graphs on innerfidelity or headroom. These report to be the most "balanced" across the board.
The fact that these HD681 are open vs the closed K271 doesn't change the FR, but they probably react better than if they were closed. Despite the physical external design resembling like the k240, the HD681 don't sound too close to it and is very close to the k271.
The drivers are regular looking 50mm diaphragms and are not in any way like the ~30mm varimotion diaphragms the k271 has. They probably don't resolve as well, but sound amazing and are some of the clearest phones I've heard.
The headphones, being packaged in a weird cardboard cutout, have indents on the soft surfaces from the edges of the cardboard. The pads for example, resembling SRH840 pads in shape, have two lines pushed into them due to crummy packaging/shipping (free shipping from China - purchased on eBay). I am sure they will quickly smooth out. They are a stiffer rubber but make a good seal that isn't uncomfortable. These phones may benefit from SRH840 pads (model HPAEC840). Shipping from China to Canada took approximately 20 total days. The order was shipped 5 days after I paid.
The 1/4" to 1/8" adapter is screw-on and appears to be of decent quality. There is no auto-mute and the clamp force along with headband expansion stiffness seems good. When I first got them, there was some white power manufacturing residue near the edge of where a pad meets the headphone. This is easily
wiped off but still I took the pads off and quickly rinsed/dried them.
I personally will not be applying the filter mod as I prefer the speed and detail. Highs register and temporally position the remainder of the spectrum for me and I find I am too sensitive to mids to enjoy any honky sound headphone. All subjective though. I can only imagine things will get better with burn-in. I'll write about any drastic FR changes if they occur.
All in all, for 35-40 CAD shipped this is an AMAZING unit. Great disposable phone for when you don't want to worry about losing high end units; or, these may be a real budget panacea for some.
TL;DR: Look at frequency response graphs comparing the two superlux, with headphones you know well, to find which you'll prefer more.