I didn't get the headphone "soundstage" thing, my HD650s were pretty much - left, right and middle.
I was on eBay when I saw a headphone amp on auction with no bids so I bid a few bucks which came out at $1.50. I didn't notice until I had committed my bid that postage was more than $40 - oops! should know better.
Well I thought that's not much more than the components for my O2 cost and when I looked at the specs I realised that the 4 chips (2xAD797 and 2xBUF634) alone would cost me about that so I felt a bit better about the cost. It was described as SR71A XP7, it didn't look like either of those RSA units so I guess it's more of a "homage" than a rip-off.
Got it yesterday, spent nearly all of today listening to all sorts of music (CD FLAC rips > ODAC).
Now I get the "soundstage" thing.
Love my 650s more than ever.
Had a similar experience with HD600 when I changed OP-amps on Xonar ST, not that the soundstage was bad before but it got way better!
edit. the soundstage improvement is very hearable through loudspeakers as well!
Edited by xeizo - 4/27/13 at 1:32am