Just contributing to the impressions thread.
With the W4/iPhone4 using the headphone out, I can hear the hissing people are talking about, but it's not loud to me. It's very silent and I would have to concentrate a bit to hear it. Playing music, I can hear what the amp does...but it's not to my taste. The bass is definitely more impactful, but it doesn't sound sharp or clean. It feels very round and it doesn't feel very emotional. I've been listening to a variety of genres and I will just shoot some stuff out about them generally. My descriptions will compare the W4/iPhone4 combo vs the W4/ZO2/iPhone4 (lowest SVC setting) combo.
Jpop/Kpop: This amp works with Jpop pretty well. When I listen to Jpop, I like that the bass hits strong and for some reason, the bass doesn't wash over the vocals. I think this is because the vocals are a bit trebly to begin with so it balances out. Jpop is one of the few genres of music that will make me boost SVC up 1-2 levels for more fun. Overall, a positive experience.
JFolk: Female vocals have much more body but when the percussions come in, it overpowers the vocals so it sounds a little messy. I think the bass is too strong and ruins balance. Overall, a negative experience.
House/Electronica/Techno/Brostep: This is another genre where it's nice to bump up the SVC 1-2 levels to get more fun out of the music. When listening to Kaskade's Here & Now album, a really trebly album, the ZO2 tones down the treble and makes a long listening session much more bearable. This goes for the other genres as well that also have spiky treble. Normally when I listen to these albums, I have to turn down the volume because of the treble, but I lose the impactful bass as well. With the ZO2, I can keep both and be happy. Really fast songs do get a little messy, but the bass makes up for it I guess. Overall, a positive experience except for really fast songs, then it is a neutral experience.
Amer. Pop: Same thing as Jpop, but usually the vocals aren't as trebly for the females. With Ameripop, I find the voices to be a bit too thick for my liking but it's only a problem if the voice is really deep sounding. If it's a relatively normal sounding female, then the ZO works fine. Overall, a semi-positive experience.
Pop Rock: I apologize. This is all I listen to from the rock genre. The snares don't hurt my ears now, and the kickdrums hit really strong. The bass guitar has more presence but the combination of the kick drum and the bass guitar increase hides the power of the electric guitar in most songs. It's too thick for rock. Overall, a negative experience.
Trance/Downtempo: I think this amp works well with this genre. It does have a signature that helps make the music more mellow. The percussions and piano have a nice weight behind them. I don't notice any hissing noises in the quieter passages, so that's pretty good. Overall, a positive experience.
Hip-Hop/Rap: If I'm listening to the more modern stuff, then the amp helps tone down the sibilance and hides some of the awful sound quality. Spoken word is pretty good, very authoratative sounding, and it makes older rap songs much better. Overall, a positive experience.
Jazz: The amp does take away a lot of noise in the not so good recordings. Surprisingly, the ZO2 doesn't overpower the bass coming from the drums, so it doesn't spill over the other instruments. It's just the right amount at the lowest setting. One setting higher and everything is ruined. The other instruments come out fine, but I feel like they lack a bit of energy. Also, the background piano requires more concentration on my part to follow it. I'm not sure how I would rate it...but I guess it's somewhere between positive and neutral.
Classical: When the percussion plays, it seems to really shock the living hell out of you...but that's not worth it when the percussion overpowers the other instruments. Even with the Westones, it was really hard to separate the instruments when there is a very busy passage happening. For slower classical pieces, I think this amp would work. For faster crazier pieces, no. Opera singers have more presence, but it gets a bit too busy when everything is happening all at once with this amp. The bass is too strong. Overall, a lukewarm to negative experience.
When phone calls come in, the hissing noise is horrendous during a call, and also, the clicking noise is present and it's quite jarring.
Would I keep the amp? At this point, no. Here are my reasons:
1. It doesn't feel like the bass is clean enough when it's added to my music. It makes some genres fun, but overall it doesn't improve upon most of everything.
2. I can't use my iPhone line out. The hissing is loud...I mean LOOOUUD. There's no way music can cover it. I can always hear it. Also, I think the volume adjustment using the Line out option is horrendous. The lowest volume is already quite loud, one volume higher and I am literally in shock from the volume and yanking out my earphones before I go deaf.
3. Usually when I buy stuff, I just listen to it. I don't really care too much for burn in. I believe in it because I can hear it, but usually I will like something on first listen. The ZO2 is fun, but it feels like it's not going to last long. I like to compare it to my Westones. When I first wore it, I liked the sound. I wasn't in love with the sound, but I liked it very much. But during that time, I had so many urges to sell them and just buy the Pro 900s again because I missed the impactful bass. But I never did. The ZO2 is not like that. I'm probably going to write a listing on Headfi to sell it after I post this up. I feel like I can get another amp, that will provide me with a tiny joy, but it will last for a longer period of time than the ZO2.
Hope this helps and you aren't too annoyed by my ranting. Let me know if there's something you would like to know and I will help you to the fullest extent.
Edit: Did all my ZO2 listening with Max ZO volume, 1/4 iPhone volume, with SVC at the lowest setting.
. Regarding SQ, my initial impression while listing with the W4s earlier today was that there was a little loss of clarity as the contour setting is increase and the bass was a bit muddy. However, it has seem to clear up ever so slightly, although that might just be the change in listing environments (I was at work earlier, now at home). I am not a big believer in burn-in but have heard it happen with static hardware before, specifically DAC that are know to have some warm-up/burn-in changes. Since I have no idea what is actually in this little thing I am not going to assume that it doesnt contain anything that would possibly be impacted over time. 
