[flixering]

Which of them is better? The schematics are nearly identical, except the WM8524 stage on the GrubDAC. But does it make a huge difference in the sound quality? Has anybody built/listened to both of them? 
(sorry for my English)

 

[Avro_Arrow]

The GrubDAC has a sound quality advantage over the SkeletonDAC.
The SkeletonDAC has the advantage of a S/PDif output that can
be fed to another DAC.
 
A more fair comparison would be the GrubDAC vers the PupDAC

 

[flixering]

The pupDAC is quite interesting, but the pieces are too expensive for my needs. Could any of this dacs drive a headphone with 40 ohm impedance and 100dB/mW, at decent volume? Without any opamp, or anything, just the audio output of the PCM2706(skeleton) or WM8562(grub) or PCM1794(pup1).

 

[Avro_Arrow]

None of the three DAC were meant to drive headphone directly.
They all sound best when used with an external amp.
 
That said, they will all drive efficient headphones at
lower volume. Don't expect them to deal with more
challenging headphones or noisy environments.

 

[tomb]

Personally, I think the comparison in quality goes something like this:
SkeletonDAC < grubDAC << pupDAC.  I believe the designer of all three - cobaltmute - feels this way, too.  We go to great lengths to try to match up cost with proportional quality at Beezar.

 
The pupDAC has an opamp for its output, so it will do much better driving a headphone directly than the other two. Keep in mind that it's still USB power, so the voltage and current will be limited. For an incredible treat, though, try listening with some quality IEM's directly from its output.
 
The SkeletonDAC has a headphone amp built into the newest PCM2704/5 chips, so it will do well, too.  The problem is that the SkeletonDAC depends on filtering capacitors at the output and you will lose bass with low-impedance phones unless you make the output capacitors large enough.  This has to do with the bass-filtering effect of the RC circuit formed by the headphones's impedance and the output capacitors' capacitance.  An RC circuit has a calculable cutoff frequency at the low end.  Since you can't easily change the headphones's impedance, you need to upsize the caps.  I have powered KSC-75's using 47uf, 6.3V capacitors on the output of a SkeletonDAC for extended periods.  It sounds pretty good, although the bass sounds a bit artificially tight, probably due to some artificial filtering in the lower frequencies.
 
The grubDAC has neither an output opamp nor does the Wolfson DAC have a built-in headphone amplifier.  IMHO, it will probably do the worst with a direct headphone connection.
 
As Avro_Arrow points out, all three DACs are really designed to be sources, not drivers of headphones.

 

[Avro_Arrow]

I have 220uF 6.3V Silmic II on the output of my SkeletonDAC.
Sounds pretty good with my Shure IEM.
 
Of course my PupDAC sounds much better...

 

[cobaltmute]

The grubDAC has neither an output opamp nor does the Wolfson DAC have a built-in headphone amplifier.  IMHO, it will probably do the worst with a direct headphone connection.

I would agree with this statement. IIRC correctly, the WM8524 wants to see at least a 2KOhm load. That said, I have driven headphone with it. It works, but you are not getting the best sound.