I'm going to do a very quick review, so hang on:
Design: Well, it's kind of hard to decide because of the difference between the headphones. The Koss Porta Pros are famous and likeable because of their design. They are thin and very sleek, and it's really easy and cool to wear them outside. The M-Audio Q40's look sturdy and kind of "manly" if you will, but if you are going to wear them outside they are kind of robust and really big, but for what they are, they are gorgeous.
The Denon is really sexy and sleek, but it looks so fragile. But that's the price you have to pay for the design, I have gotten compliments for this headphones, well about their looks.
The winner: Denon
Comfort: Koss Porta pro is obviously going to be comfortable and light, so you know what you are getting. The problem that I have with them is that they slip of if you run or jog, and if you have a lot of hair they can get tangled in them.
M-Audio Q40 are kind of comfortable, but the clamp you head quite tightly if you have a big head. The pressure becomes easier if you stretch the headband, but all in all it really cups the ears and it feels like you ears are in a separate room. Well, I wouldn't recommend them for outside use or for long hours. I get sometimes really hot around my ears and a slight headache after using them.
Denon is hard do describe. The pads are really horrible, they feel cheap and it's to thin. If you have short hair or a big head then the top of the headphones are probably going to scratch and put some pressure on the top of your head, but that won't be a problem if you have long hair.
The winner: Koss
Sound: For the price, the Koss's are great, you can't argue with that. If you are picky and a audiophile you are going to dislike them because of the lack of detail. If you listen to rock then a lot of the layers and guitar parts are going to disapear, but they are great for songs with a lot of bass.
I got the M-Audio Q40's a couple of days ago and they are good overall. The bass is powerful and always there. They are brilliant if you listen to Michael Jackson (and such), but they are to "trebly" and the sibilance is really annoying and harsh if you want a powerful sound when you listen to rock and metal. They really hurt my hears when I tried some Metallica. If you listen to songs with a lot of bass then this is for you.
The Denon sound kind of "far away" and the sound isn't really hitting you hard, but it's quite nice. It's just there, it doesn't force you to listen to the music and it really shines when it comes to the detail and layers in songs. If you like metal or rock, then this is really good because of the details. The treble is quite smooth and doesn't irritate you like the Q40's.
The winner: Tie between Denon and M-Audio. Depends on what you are after.
I was quite disappointed with the Q40 and I'm going to give them. I'm not going to say that they are bad or that I dislike them, but they are to cold and harsh for me. I really like smooth headphones with a thumpy bass and I really don't listen to a lot of bassy songs.
If you like bassy songs, they are great. The Koss Porta Pro are quite decent, there is not a lot to say. For it's price they are great, but you won't hear anything if you are in the bus or on the streets.
Denons are not so powerful and doesn't have that "wow" factor, but with a great source of music they really shine.
Used a cowon S9 with only flac music.