My reference for reviewing the HD 650 is the AKG K701. I've used the HD650 with various amps and sources including the Nuforce HDP, Headroom Ultra Micro Amp (now discontinued), the Beta 22 (2 channels), Musical Fidelity M1 and the ALO Rex MKII. Sources included Marantz CD5003, Ultra Micro DAC (discontinued), Ultra Desktop DAC (discontinued) Nuforce HDP and Centrance DACPort LX.
 
I think many reviewers have already talked a great deal about the characteristics of the HD650. Most of them have better experience and probably are more capable of discerning the pros and  cons. I only wanted to add a few points regarding classical music listening.
 
Some listeners shy away from the HD650 when listening to classical for their dark sound and lack of airiness but for my ears they are suitable for the genre. First of all they have a beautiful natural tone for strings and winds, in comparison to the K701 which has an anemic neutral to bright tonality. The HD600 has a similar tonality to the HD650 but IMO the latter is more refined, less grainy and enjoys a better sound stage and imaging. The only drawback is the HD650 need for a neutral to bright system to sound their best. With some set ups it can sound too dark and dull. I heard them at their best with the Beta 22: a very big and tall soundstage with exceptional depth and detail. 
 
When listening to orchestral music on the HD 650 there is more weight and impact. The experience is closer to live performances in comparison to K701. There is also more emotional involvement, using the K701 is like reading the notes, while both HD600 and 650 reveal the mood and the feelings of the piece. In other words, listening to the HD 650 is like being in the concert hall, while the K701 experience is closer to being in the studio. 
 
The HD650 is more forgiving with older recordings. Classical gems like Arrau's recordings of Beethoven's piano sonatas or Oistrakh glorious recordings of Prokofiev and Shostakovitch can't be rendered with revealing headphones like the K701 or the HD800.
 
It's also much easier to match the HD650 with the right DAC and AMP in comparison to the K701. Actually the HD650 is an excellent performer with high grade equipment and a steal at its current price (around $300 used). For the K701 it's very hard to find the right amp to pair with it, it sounded at their best on the Luxman and MF M1 HPA amps. 
 
The K701 is definitely more airy, with a wider sound stage, but the warm tone of the HD650 provide a solid placement of instruments in space. Sometimes I feel the K701 too thin sounding for complex passages compared to the HD650. 
 
This review was done on mid fi gear involving Cetrance Dacport Lx and ALO RxII. 
 
Update 03/28/14
 
I did an extensive comparison on the current set up: Flac and Aiff files played by Audirvana plus on a Macbook computer. The macbook is connected by optical cable to Headroom Ultra Desktop DAC. The UDAC is connected to Musical Fidelity M1 headphone amplifier via DiMarzio RCA cables. 
 
On this set up the AKG Q701 is performing very well indeed. The highs are smoother and the sound is more open, with fuller, richer and clearer mids. The imaging and the depth of the soundstage have been immensely improved. In short I feel that the Q701 and the MF M1 amp are very good combo, much better than the HD650 combined with the same amp. Probably the M1 being slightly warmish works better with brighter headphones like the Q701 and the HD800. 
 
The HD650 is certainly more fun to listen to, but the Q701 is more detailed with a wider and bigger sound stage, better imaging and more extended treble. But the tonality of the HD650 is more true to life, where K701 sounds pale in comparison. When listening to the HD650 you feel like living inside the music, while the Q701 is more objective and cold. The mid on the HD650 is rich and has a palpable presence, compared to the thinner mid on the Q701.
 
 
Of course these observations are purely subjective, and based on my experiences with different set ups. 

Both K501 and Sennheiser's HD600 are the sweet points of Head Fi. They are sold at a fair price and they do nothing wrong, they only differ in the way they reproduce music. While the HD600 is soft, smooth and mellow, the K501 has more detail, transparency and more extending treble which is never harsh or piercing in comparison to the K701. The mids of the K501 are magical in comparison to the K701 which is a bit recessed. There is a sense of immediacy with the K501 which brings you closer to the music, a feeling I never had with the K701. It's a pity that this headphones is discontinued, I wish AKG would create a similar headphone with better bass extension and detail, better separation of instruments and better imaging and keeping the same tonality of the K501. The K501 sounds more natural than the K701 which I never enjoyed in the 2 years that I owned them.
 
Update 01/09/12
 
I acquired a used pair of K701 which has been used for 1000-2000 hours. I really enjoyed this particular pair more than my K501 which I eventually sold to another Head Fier. The K701 I own now has better detail than the K501, more extension in treble and bass and wider sounstage. The K501 has more coherent imaging and more forgiving with badly recorded music than the K701. I hope that helps!

I bought the AKG K340 Electrostat-Dynamic System twice. The first pair was very worn out and sounded weird so I sold them after a while. I bought the second pair from Ebay, and this one sounded very different. I used the first pair with the Beta 22 amp which really struggled to drive it properly. The second pair I'm using with Headroom Ultra Micro Amp with Astrodyne PSU, and surprisingly it drives this pair with no problem, I hardly pass 10 o'clock of volume!
 
I suspect that the second pair is the bass heavy version. There is a lot of bass here with deep impact, although not very detailed but not loose bass like that of D2000/D5000. It's well integrated into the spectrum of sound and adds a lot of warmth to the sound character. 
 
The Mids and Highs are the best quality of this pair. They are very clear, very detailed and very smooth. This is attributed to the double driver system of the K340 (electrostat and dynamic), which eventually results in a very layered sounstage of 3D holographic nature, while on the K702 it feels 2D in comparison.
 
Compared to my beloved K702, the violins and flutes sound very smooth and extended, while on K702 they seem very bright in comparison with slight harshness. If I don't compare the K340 to K702 and keep listening to the K702 I don't feel there is any problem with the highs, they look absolutely OK, but when I put the K340 on, the K702 pales in comparison and sounds gray while the K340 is all black and white. The only area where K702 excels is a wider soundstage (which lacks in depth) and the bass is more detailed but without the impact and warmth of the K340.
 
On listening to DG's marvellous CD "Fiesta" or Boulez excellent recording of Varese's Arcana on Sony, I feel like a real Orchestra is playing on the stage, giving the instruments a great weight and impact that I never felt with the K702. Playing Beethoven's Cello Sonata No.1 by Schiff and Perenyi  I was overwhelmed by the Cello's deep and dark sound, it was a great emotional experience that brought tears to my eyes!
 
On listening to the K340 I only think about the music and the composer who created it, while on listening to the K702 I only think about the good sound quality! 
 
Considering their price range of 150 to 200 those cans are a steal, they can beat HD650, HD600, K702 and even my recabled K400 in performance. They are not perfect but considering the price and value they are a real treasure!

I owned the Nuforce HDP for two months, I tried it with several headphones ranging from the 600 Ohms DT880 to the 300 Ohms Senn HD600 and HD650 and 62 Ohms AKG K702. Actually I wasn't impressed that much. I didn't feel that the amp had enough juice to drive any of the headphones mentioned above, and the DAC had too much emphasis on treble which seemed too exaggerated and unnatural to my ears. When listening to several audio tracks and in comparison to Headroom's Ultra Micro DAC I heard distortions in the upper highs that never showed in the Ultra Micro. The imaging and accuracy of the UM DAC was far beyond the reach of the HDP, but on the other side, the HDP performed better than my Marantz CD5004 regarding clarity and detail. The best sound I got from the HDP was from the Marantz CD player connected by coaxial/ optical connection. But USB connection sound quality was thin, harsh and lean. 
The volume knob is made of cheap plastic and sometimes I heard static when moving the knob between 1 o'clock and 3 o'clock. On the other side, the HDP is very small size, very portable and it has many connections on the back which makes a great versatile unit for many purposes.
So all in all it is not a giant killer. It doesn't function as good as DACs or Amps double or triple the price, but you get what you pay for. A $200 DAC and a $200 Amp no more, no less.

The HD600 is one of the best headphones around. They are neutral, easy to drive, comfy, with a touch of warmth and enticing natural presentation and timbre. I didn't like them the first time I bought them with the stock cable. I bought another pair driven by the formidable Beta 22 amplifier and the HD600 was transformed into another league. 
 
Although I listen mostly to my K702 which is more accurate in comparison to HD600 warmer tone (which is closer to concert hall sound IMO) I save the HD600 for very special listening sessions, I don't use them for occasional listening at all but only when I really need to enjoy rather than analyze my music.
 
The highs are soft, smooth, non fatiguing. The mids are large and wide and the bass is just right, without being exaggerated. The sound stage is not very large like the K702 but the instruments and the performers are very close in very good distributed space which gives a very intimate musical experience.
 
The greatest thing about the HD600 is the easy amplification. A uDAC can drive them to very acceptable level and they can excel with most available amplifiers with a neutral sound signature
 
Some reviewers believe that the HD600 is boring. I think this can be attributed to several factors, poor synergy with amp and source, stock cable, lack of burn in or may be personal taste!
 
IMO the HD600 is the best headphones under $300 (unless you find the HD650 for less than 300!), and may be the only headphones you will ever need if you keep to one HP.

This amplifier didn't drive my ER 4S earphones properly neither my AT M40fs! May be it's a synergy issue, but I'm not sure, at least nobody complained of ER 4S being hard to drive! The sound was too dark for my taste, with a collapsing soundstage (compared to my Onkyo receiver) and very poor extension in the highs and lows but the mids were good. I used it with my Marantz CD5004 connected by Blue Jeans RCA cables and this combination was awful! I guess the dark Marantz is not the best combo with the dark caliente. In the end I think there are many better choices than the KICAS Caliente for the price range of $300. May be it works fine with the AKG K701/2 but never tried it with those cans.

The K701 is not the most perfect headphone, but at this price range it can surely compete with headphones double the price. They certainly need a powerful amplifier to drive them properly. I never appreciated them truly till I used them with the Beta 22 amp and then I found out what I was missing on the lesser amps I used. The best K701 I heard where used around 1000 hours. They sounded very smooth, relaxed and a joy to listen to. They can pick up any weakness in your rig: Source, Cables, Amps. They need careful matching to sound correct. Compared to the K400 which I also own. The K501 has a beautiful midrange which gives a very intimate and special experience, its more enjoyable and more musical, but the K701/2 is more accurate, more detailed with more extension in the highs and lows but sometimes can sound sterile and boring. The K501 lacks the detail and extension of the analytical K701/2 (although they are not lacking in either) but listening to the K501 brings the music to life with a rich tonality which is missing from the K701 with some tracks.  

I'm surprised that no review has been written about those gems! The ER 4P/S is one of the best IEMs in the market value wise. They are made to last, they isolate like no other IEM and they have a unique flat frequency response which can be easily adjusted if you have a good equalizer or an integrated amp. The ER S will sound best when used with a powerful amp, it will provide an amazing sound stage and wonderful imaging of instruments and players. They can provide life like details of the recording including movements, coughs, breathing and pages turned over depicted with extreme clarity. Highly recommended.

I'll keep my comment brief and short. The sparrow is definitely not the same as your ipod or PC sound card. It's an improvement over both. I bought the A version and I use it with my Shure SHR 840 with good results. The sound is rich and full with decent imaging. For the price I paid no complaint. Audio GD were extremely patient answering the questions I showered upon them, I recommend this combo for sub $150 set up, but I believe that the uDAC is the best deal to have in this range.