Sennheiser HD 650

Pros: comfortable, smooth, lush and thick sound

Cons: a bit slow on non balenced amplifiers, not as clean as K701

Summary: I purchased this headphone (new) in late November of 2005 at a cost of 290 dollars...these days it could be found for around 310 to 400 on the internet and i've owned it for 5 months now and have put in about 2000 hours on it (or more).
At first, my reaction wasn't that good...i did make the mistake of plugging it into my ipod where i listened and found little new; my PX100s gave me nearly identical sound. Two things happened here: first was the fact that these headphones were not burned in-they open up after about 150 hours of "burn in" time, and secondly my ipod's DA converter and internal headphone amplifier wern't good enought to show me what this headphone really had to offer.
I went out, turned my tunes into lossless, got KSIO drivers and a new sound card in addition to an amplifier for them (Meier Corda HAI MKII, now a Cordafive). there are reasons to do this: first of all these headphones are highly revealing of any mistakes in the music...if you could't hear the difference between 128kbps ACC and CD quality on the ipod's ibuds you will on these...the sound difference is striking. Secondly, the DA converter has the capacity to either make the sound harsh, soft, wide, impactful depending on how good it is and how it is converting music (for example a Non oversampling dac may increase the frequency range and give you better bass while an oversampling da converter may give you a better soundstage), and with the HD650 any little change is noted. Thirdly, the amplifier has to do with the dynamics, even though my ipod could power it up to as loud as i wanted it, the ipod simply does not have the speed or power to give me a realistic volume increase from a piano to triple forte, the ipods' dynamics are simply limited.
I guess as you have probably noticed these headphones are for serious listening...yes the ipod will sound as good as it possibly can but that left me wanting more and more; thus i got "upgraditis" and tried to fix all of the limitations in my music chain (from CD to headphone) that might degrade the sound in my quest for ever better music: and the HD650 improves with every single upgrade.
The cable: the included cable on the HD650 is 9 feet long, is two sided, and makes no noise. included is a 1/4th to 1/8th connector that's pretty good. One of the things about the HD650 is that because the cable is so easily replaceable and switchable in less than a minute a number of third party manufacturers have started making upgraded, higher quality cables. these are meant for the serious listener who wants even more...and at 90 to 250 dollars, these cables only give one a marginal increase in performance/resolution and alone arn't worth the money you pay. nevertheless, in my quest i did buy a new upgraded cable, the Blue Dragon V2 from moon audio which tends to refine the bass and flatten the frequency response a bit; but there are others that perform just as well: the Cardas headphone cable, the Zu Mobis and the Equinox Sennheiser cable.
The confort: at first there is quite a squeeze on one's head, but as it adjusted over the months it came to fit my head perfectly, also the pads soften up over time to make the fit feel like one is wearing a pillow on one's head.
Portability: now this is interesting; normally one would think these headphones are too big to go around...but they can if one is willing; there are a number of portable amplifiers that can be connected to an ipod from a pocketdock and a mini-mini cable that would make this an excellent portable combination.
Sound (finally): The sound on this thing is rather bassy, if one was to think of it as a piano then the lowest notes would be the loudest, followed by the middle and the highest notes would be crystal clear but not as loud as any of the other notes. The sound is very smooth, warm, and extreemly lush...not completely neutral but not distorted either, just perfect for music enjoyment but not neutral enough for studio monitoring. Compared to normal headphones it's superfast and thrilling, yet when compared to say a Grado RS1 then one realizes that it's not as fast and rock music doesn't sound as good as it could. The way the music is presented to someone it's as if one was sitting in the middle of a concert hall-in the perfect seat; your not in the band, upfront like in a grado but that certainly doesn't make it boring. The soundstage is rather wide, and can be precise depending on the gear down the chain; those of you who havn't listened to open cans of this type could think of it as the ability to listen to music and point to where a particular insturment is seated and how far away someone is singing from you. If one were to listen and think of it as music "in your head" then the 3blob affect comes in and one would notice that there is music comming from right, left and in the middle but not left-miffle and not right-middle; there are exceptions, every so often i get a piece of music that defies this but this is what occurs 80 percent of the time-a good cross feed though can dampen the effects of this. Overall the music listening experience is very enjoyable and capable of entertaining one for long periods of time.
Problems: First, the volume, as iv'e mentioned, of the treble is sometimes sited as a weakness while others say that it helps one not become fatigued by the sound. with my new cable i've found this volume dip at the bottom to have disappeared, but that has also revealed the harshness of my soundcard. Secondly, there is the "veil" this is characterized by a lack of clarity in the midrange, which is smooth and beutiful sounding but maybe not as loud and beautiful as is found on other headphones....this actually tends to go away with higher levels of amplification...but it takes maybe a bit too much money to make this problem go away. thirdly we have the open design; because of the way these headphones are engineered ambient noise isn't blocked out, so one has to seek a quiet location in order to maximize their listening experience.
finishing thoughts: overall, these headphones are actually responsible for introducing me to higher levels of listening. five months ago i was content with just the ipod+PX100 combo, thinking that sound simply couldn't be reproduced to realistic levels, now i realize that it can.