My particular model is the K 81 DJ.

I bought this pair of headphones after extensive research and reading on sites and forums around the internet. Eventually, I chose these based on the fact that A) They can gut out the bass, B) They can be loud, C) They can sound very good, D) They attenuate in/out noise, E) They present music in an "exciting" manner.

When they first arrived, I decided to hook them up to some heavy bass tracks and left them at a lower volume for a couple hours. Then I turned the volume up and let them go with a very bassy track on repeat for 30 hours. After this, I listened (as objectively as possible) and noticed they were pretty balanced sounding.

As I listened to many different types of music, I found these to be pretty good sounding, except I had a problem: The bass seemed pretty weak.

So, what did I do? Well! At the time I was powering them with my portable radio, which got them loud, but the bass was lacking. In fact, Frankenstein by the Edgar Winters Group was playing and I turned it up to make the sound fuller. Someone had me take them off my head and everyone in the room could clearly hear the music.

Although these headphones can be loud, they seem pretty lacking in bass (but I am a basshead). This led to me building a headphone amplifier, called a CMoy. The original design is a simple amplifier that just takes a signal in and provides power for the headphones. I modified the design instead and added a bass-boost circuit into it, which adds more and more bass starting at around 70hz (3dB boost) and tops out with a whopping 25dB boost at 23hz, going back down after that.

Afterwards, I hooked it up to my portable radio with the bass boost off, and already noticed a bass improvement. However, it was even better with the boost on. Off of a cd player, these headphones also sounded quite good. However, I am not sure if it's the amplifier or headphones, but the headphones can't seem to handle much bass without sounding dirty (again, it may be the amplifier). I normally don't push the headphones louder than 100mV, or 400mV for bass. I listen to a lot of metal on these and they do a good job, and the bass isn't too bad either.

Off of my A/V receiver (Pioneer Elite), these also sound great, but the bass still seems to have limits.

Overall, these headphones are great, but they can be light on bass (for a basshead), but otherwise sound pretty balanced. They don't seem quite as sensitive as they say they are. Highly recommended for the price and the sound is quite good. Classical music sounds great as well. An amp is not generally needed, except for bass-heavy music.