The Superlux HD 668B are great-sounding and affordable critical listening headphones. They let in a lot of ambient noise and leak enough to be distracting to those around you, so they won't perform as well for other use cases. They're also a bit bulky and cumbersome to carry around. However, when it comes to sound, they deliver an almost unmatched audio reproduction at this price range.
The Superlux HD 668B are a good, budget option for neutral listening. They deliver a satisfying and tight bass that doesn't overpower the presence of instruments and vocals. They have a relatively flat mid-range but a slight bump in the high-mid /low-treble that makes them sound a bit sharp on some tracks especially with songs that are already a little bright. Sadly, they don't have a soundstage as immersive as some of the other fully open-back headphones, but overall the sound quality of the HD668 is sufficiently good for both casual and neutral listeners.
Subpar for commuting. They're not intended for this use case as they don't block a lot of ambient noise, they're not portable and do not have the most durable build quality.
Not made for sports. They're bulky and impractical to use outdoors. They're also not stable enough to use while doing physical activities and will slide off your ears if used while running or working out. Also, they don't have any controls, which makes controlling your music source while exercising a bit more difficult.
Below-average for office use. They let a lot of noise seep into your audio and also leak a lot which might be distracting to your colleagues.
The Superlux HD 668B and the Superlux HD 681 are both very good headphones if you like a neutral sound, but the HD 681 have a slight edge. While they both can sound quite harsh and sharp, the HD 668B also lack thump and rumble, on top of sounding a bit muddier. The HD 681 are also much more comfortable.
The Superlux HD 668B are somewhat better headphones for neutral sound than the Samson SR850. They're very similar-looking semi-open headphones, but the Superlux have a much more accurate treble response, so they don't sound as sharp or piercing. Their audio cable is detachable, which is convenient if it gets damaged and needs replacing. They're also more breathable, but you may find the Samson's leather strap headband design more comfortable than the Superlux's cushioned headband.
The HD 668B look very similar to the AKG K240. They have paddles instead of a fabric strap to cushion the headband and an odd-looking male audio jack in place of a more traditional female port for the detachable cable. However, the rest of the HD 668's design, including the large semi-open ear cups, look like a cheaper version of the K240s. Overall, they have an understated and utilitarian appeal but they're not headphones you would use outdoors for their style.
The Superlux HD 668 are decently well-padded but a bit too tight on the head. They're lightweight and the ear cups are large and fit well around most listeners ears. Unfortunately, the headband doesn't extend to accommodate different head sizes, so they tend to feel a bit tight. That and the paddles that act as a cushion for the headband put quite a bit of pressure, which will get fatiguing after a long listening session.
The Superlux HD 668 are cheaply made and plasticky. They feel like a budget version of the AKG K240, which shows in their build quality. They have a plastic design with no metal parts, and all the materials used in their build quality feel slightly low grade. They won't break if they accidentally fall once or twice, but they don't feel like durable headphones. The paddles, especially, are weak points that will wear over time.
The HD 668B are tight on the head which make them decently stable. They won't fall during casual listening sessions and may even be tight enough for a light jog but they are not sports headphones. The earcups are a bit bulky and move a lot under physical activity so they won't be the ideal headphones to go running with. On the upside, the detachable audio cable will disconnect if it gets hook on anything
Good consistency. The Bass Range of our Over-Ear and On-Ear headphones are measured on 5 different human subjects, 5 times each. In the graphs, each line represents an individual's average Bass response. The variance in the Bass range is very little, except for one of our test subjects who wears glasses, which gets about 3dB less Bass at 40Hz. In the Treble Range the variance is about +/-3dB is which is decent.
Poor Isolation. Given the semi-open design of these headphones, poor isolation is natural. These headphones don't block any noise below 1KHz. Above that, they achieve about 20dB of reduction which actually is above average.
Poor Leakage performance. The semi-open HD 668B leak less than some fully open-back headphones, but more than the ones with a closed-back design. The significant portion of their leakage is between 400Hz and 10KHz which is a relatively broad range, and the overall level of the leakage is also fairly loud.
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