Got the BUF634 buffers installed the other day.
Both BUF's are installed up-side-down, each has two pins soldered to the AD8599 pins, so they are mounted quite securely machanically (the SOIC-8 package doesn't weight anything, with two direct attachment points they are not going any where ). All jumper wires have teflon insulation, I think power wires (the yellow and grey) are 24awg and the white signal wires are 30awg.
The whole mod took a lot longer than I expected, about 30 minutes on figuring-out/planning the mod and another 1.5hr for the actual operation. If I have to do it again, I suspect it will still take at least 45 minutes since everything has to be done quite carefully, including folding some of the buf634 leads backwards. A magnifier of some kind is an absolute must if one want to do the mod, ideally you should have access to a stereo microscope (which I did use, very helpful during the soldering).
I took a lot of step-by-step photo's thinking I mgiht want to do a tutorial, but at this point I don't think they are needed. I have to admit I don't like the sound, at least for now:
-- with sensitive phones (such as my EarSonic SM3, which sounded quite groovy driven by just the AD8599), the sound seems to be harder and missing the "feel" of music (missing low level details, I suspect), and it is a bit too bright....and this is with SM3 which is a supposedly dark-sounding IEM....
--for the more power hungry phones such as T50rp, the buffered circuit has a bit more power than before, the bass for T50rp is now quite ok, but the sound has a certain brightness/hardness to it. And it might be less detailed (don't remember how detailed the T50rp was with the original AD8397, it was a while back and all I remembered was thinking the T50rp's bass sounded too soft with the stock X3; AD8599 without buffer couldn't run the T50rp well either ).
Maybe the new buf's just need more burn-in. The X3 is now playing in repeat mode with charger plugged-in, I will let it run non-stop for the next 2~3 days and then take another listen.
But I suspect the bright/hardness of the buffered sound is due to the x3 power supply can not keep up with the now much larger power demand. Maybe I will have to add at least some larger power caps. Not sure where to mount them yet.