First, off the Superlux HD-668 B are a great value for what you get. I recommend modding them for better listening sound and comfort.

Mods:

• Added electrical table inside the cups covering the holes. This lowers the sound stage but not too much and helps control the sound.
• Removed paper inserts in the cups because not needed with 1st mod
• Removed foam inserts. Clears up the sound, helps to tame the treble somewhat, and also, bringing the mids forward.
• Replaced stock earpads with Tranesca leather Angled big overear earpads. This gives comfort over the cheap stock pads
• Resoldered the poorly done wires for better lasting connection.

Fixed the clamping by stretching the headphones out.

Testing:
Equipment Used: SMSL sApII headphone amp running through an SMSL M2 USB DAC using Digital Output mode in Linux with PulseAudio.
Method: I listen to a wide variety of genres from Jazz, Rock, Metal, Electronic, and Classical/Orchestra. So I tested a bunch of songs from each different genre and they all sounded great without anything missing from them.

Sound:

• Treble is way too hot and has a harsh sound. To fix this is to trim back the 4khz - 10khz rage in an equalizer but I have fixed this somewhat by modding these cans, but some EDM songs not all can have hot/harsh treble if mids/lows aren't playing at the same time.
• Mids are not pronounced and are covered up by the harsh sounding Treble. This can be fixed with an equalizer or mods.
• Bass is punchy and not overpowering that it will blast your head off.
• Lows are not missing anything in their range. They sound accurate and are very responsive.
• Vocals are very clear and natural.

Build Quality