A few thoughts about the 650, just in case you haven't read enough about it already. :)
Just to explain where I'm coming from, I have no interest in dedicated headphone amps. I don't think they're worth the money and trouble; none of those I've heard has impressed me. However, whatever type of amp you use it's important to choose one with opposite characteristics to the 650, and since the 650 is slightly on the soft, fat and rolled-off side of things we want an amp with a lean, rather bright character, with plenty of treble extension. I recently set out to find such an amp and, after reading many reviews, decided on the Cambridge Audio 340 SE. This turned out to be a spot-on, true synergistic match and had a further benefit in tone controls that only affect the frequency extremes. So, not only is the 650 brightened and sharpened up by the characteristics of the amp, it's possible to use the tone controls to add extension to the frequency extremes without in any way affecting the midrange. Thus at around three O'Clock on the treble control (which Cambridge list as being +7db at 20khz--very unusual characteristics), the 650 lacks for nothing in the treble department; there's no hint of a veil, yet the sound remains beautifully smooth. Similarly with the bass control (+7db at 50hz) at the same setting the 650 gains a low bass slam you would not suspect it of possessing, and there's no hint of "fatness". In fact, driven like this, I can't see that the 650 lacks for anything. I've not heard the HD800, and I'm sure it's wonderful on great sounding CDs, but the consensus seems to be that there is a slightly fatiguing element about it on less than great CDs. This is not the case with the 650 as I'm using it here, yet I never feel I'm missing any detail in any area, especially treble.
Maybe I'm not perfectionist enough, but I feel with this combination I've come to the end of my (rather long--40 years!) headphone journey. With a half-decent CD the 650 can present me with a believable illusion of a symphony orchestra without a hint of fatigue even at realistic levels. And even not-so-decent CDs sound better than I've ever heard them. Plus the 650 is incredibly comfortable once the initial clamping effect wears off (only takes a few weeks). I'd urge anyone looking for a balanced, neutral sound, without false brightness or thumping bass, to check out this combination. For not that many $s I think you might be surprised at how much you get.
Edited by pp312 - 8/26/10 at 4:10am