Another take with the opamps I've used:
I first gave a good listen to all the buffers and I ended up choosing the discrete buffer. The differences among (same topology) buffers were not that great
but even with my headphones they were noticeable.
The LT1010s were almost disappointing apart from the fact that they are rated to only 100mA which is marginal for driving headphones appropriately IMO.
I mean, c'mon there are even opamps that can handle 100mA loads. And since I wanted a headamp that can handle all possible kind of headphone loads
(I'm not sure about planars but spec-wise it's possible) the LT1010s had no chance.
If I had to rate the buffers according to my preference, first comes the discrete diamond buffer, then the LME49600/LME49610s, after them the BUF634, then the OPA633 and last comes the LT1010s.
Things that I like with the discrete buffers is that I can choose topology, the amount of bias and the way that it is applied (variations of current sources, resistors etc), I can also choose all the parts,
the power handling is greater than the IC buffers, there is no thermal feedback because the output transistors are not in the same package with the rest of the circuitry just to name a few.
Of course none of these would matter if it eventually sounded worse than an IC. I'm happy it didn't.
In the beginning I used the same potentiometers (miniature alpha) in the two amps in order to keep things equal when comparing buffers and opamps but after making up my mind I used an ALPS RK27 10K log potentiometer
with one of the two amps in order to squeeze more "juice" out!
I guess that will be all for tonight (to be continued)