The SA5K is a very clean, detailed, and fun phone. Lots of space between the notes, bass impact, and clarity. However, I'd have to agree with others that have posted that something is wrong with it's frequency response. This error in response is made blatently obvious when listenting to vocals, acoustic guitar, and piano. Attack and decay are all there, but lower midrange weight is missing - making these instruments sound thin and artificial. Should Sony be able to even out the frequency response to give weight to these instruments as well as tone down the treble a tad, they will have one hell of an offering. Even though I sold them, I did enjoy the strengths while I had them.
The HD650 is just about the anti-SA5K (with the exception of being clean - both are very competent here.) Piano, vocals, and acoustic guitar are to die for. They are definitely on the 'warm' side of neutral however, and in terms of frequency response alone, IMO, the HD595 does a better job. Problem with the HD595 is that it's not as refined and doesn't provide that absolute clarity, attack and decay as the 650. So depending on what shortcoming you feel is the greater error, you will prefer one over the other. I chose the 650 - but wish it to have less warmth, more in the spirit of the HD595.
Like many on this forum, I went nuts for a while. I think I'm content until the next round of offerings.
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Bought HD595, HD650, K501, DT880, SA5K.
Kept HD650 (warm, intimate, still neutral), DT880 (open, intimate, still neutral)