I recently purchased the Superlux HD681F headphones, which caused me to return the highly-rated AKG K702 that I'd bought a couple of weeks earlier, and thought people might like to hear my findings and comparison between an excellent inexpensive set of headphones and an expensive 'professional' product, especially as my satisfaction level is so much greater with the cheaper option.
The Superlux HD681 series of semi-open headphones come in a variety of three types, ie. HD681 (lots of bass, so useless for mixing - I tried them), HD681F ('F' for 'Flat' response) and HD681B ('B' for 'Balanced'), which lies somewhere between the other two. I've been advised that the 'B' version is very similar to the Samson SR850 headphones, which also have a prominence of bass, albeit less so than the HD681. As the HD681F and K702 are classified as semi-open and open headphones respectively (as opposed to 'closed' or 'sealed'), they are not good for private listening because they exhibit a lot of sound leakage. However, these types of headphone are supposed to have a wider sound-stage, so are better for positioning of instruments and expressing the stereo image. They are both bright sounding headphones, the Superlux a bit more so than the AKG, but this is necessary in order to achieve clarity and detail.
Here's a graph comparing the frequency response curves of all three model types in the HD681 range...
The AKG K702 currently retails here in the UK for around 120 GBP and has a reputation for having a flat frequency response (ie. 'neutral', whereby all the frequencies are reproduced at roughly the same level across the audio spectrum), but after a few days of constantly working with them I became disenchanted because they sounded somewhat veiled and a bit coloured due to a bump from about 50Hz to 500Hz, ie bass to lower mids, thus causing a recess in the upper mids, and failed to properly translate to my Yamaha MSP5 monitors. They sounded 'warm' and forgiving, much like hi-fi headphones, so I thought I'd try the Superlux HD681F instead, which I managed to purchase for a mere 18.70 GBP + 7GBP postage, and am very pleased with them, particularly as they closely match the tonality of my monitors, which have a nice flat frequency response with plenty of clarity. My source was a retailer in Poland who appears to only be trading within Europe, but I have seen them being sold by a number of alternative online retailers from other countries.
Now to compare the inexpensive HD681F to the expensive 'professional grade' AKG K702. They are both what many people would consider to be 'bass light', so for people who like to listen to genres of music for pleasure that have lots of bass they will disappoint, and both are completely devoid of sub-bass. However, for mixing it's not good to use bass-heavy headphones due to the tendency of excessive bass frequencies spilling over and distorting the mid-range where lots of detail lies. As far as bass response is concerned, at least according to my perception, both headphones are very similar. The K702 response curve is not completely flat due to the bass and lower-mid bump that I mentioned earlier, whereas the HD681F is more or less completely flat from 60Hz right up to around 7kHz, and in this they are more clinically detailed than the K702. If there are flaws in eq, the HD681F will expose them.
Unlike the K702, the HD681F (as with the other two in the range) has a treble spike at around 8.5kHz which can make music sound harsh, as well as accentuate sibilance. However, this can easily be tamed with a common modification - inserting two or three layers of toilet paper tissue underneath the sponge that covers the driver units. Alternatively, there are instructions on the internet for the construction of a simple passive notch-filter (much like a simple single band preset graphic equaliser but without the adjustable control) to reduce the spike whilst preserving the integrity of even higher frequencies. When I first used the one I received, I noticed the harshness of that spike. However, after subjecting the headphones to an overnight 'burn-in', playing a loop of loud pink noise plus a couple of high bit-rate 1960s music tracks (loaded into Mixcraft of course) for twelve hours, the spike appeared to become less prominent - either that or my ears adapted to it. Incidentally, after this 'burn-in', the bass appeared to sound a touch more extended, and even more so after another overnight session of bass-heavy music.
The K702 exhibits a touch more high-frequency extension that's also more delicate than the HD681F, but considering the price difference, this is a very minor detail. For all I know, similar delicate high-treble extension may be present but could conceivably be masked by the spike, but I've not built the filter so don't know for sure. When listening to my mixes through the HD681F, I found flaws that neither the AKG K702, nor my expensive Beyerdynamic DT880 and cheap Sennheiser HD201 headphones, managed to match. I realised that whilst I have ownership of the HD681F, I would have little need of the K702, and that's why I returned it to the retailer for a refund.
Here's a link to a chart that compares the frequency response curves of both the AKG K702 (red) and Superlux HD681F (blue). The latter is referred to as 'HD681' in the graph, but this is an error - it is the HD681F.http://graphs.headphone.com/index.php?graphID%5B0%5D=3311&graphID%5B1%5D=2621&graphID%5B2%5D=&graphID%5B3%5D=&scale=30&graphType=0&buttonSelection=Update+Graph
I should mention that while the AKG K702 has big velour ear pads that make it superbly comfortable, the Superlux HD681F has cheap vinyl ones that can cause ears to sweat, although velour pads are available as a replacement but may reduce bass response a little. However, at this price, who's complaining about vinyl? Furthermore, stretchable fabric covers that improve the comfort level of the pads with no noticeable loss of bass level are available on Amazon.
I think I probably got a bit carried away typing this, but I trust it will prove informative and useful.