I recently got a headphone amp (Schiit Vali 2).  For input it has phono jacks (red and white), which I connect to the Line Out 1 and 2 of the Scarlett.
 
No matter how high I turn the volume levels on the Scarlett or the amp, I'm not getting any signal on the headphones.
 
If I instead plug the amp into the Scarlett's headphone output (using a Y adaptor), then I get a signal, although it is a bit faint.
 
So my question is this: how can I control the volume level of the Scarlett's Line Out 1 and 2?

Line out 3/4 has the same problem: no output, no matter how I turn the knobs and switches.
 
The 1/4" headphone jack works, as do the 1/4" balanced outputs, which I have connected to my powered speakers.  It's just the RCA/phono jacks which aren't working.

Briefly (since there is apparently zero interest in this thread), I chatted with a Focusrite techie and found the answers:
 
1) Line out 3/4 never works outside of a DAW.  They says it is a "limitation of windows" that audio can only be sent to line 1/2, although that seems a bit silly, since Windows happily allows multi-channel output for surround-sound cards. Regardless, the rule is this: if you want to use line out 3/4 you need to use Sonar.
 
2) To get Sonar to send audio to line out 3/4, select "Focusrite USB ASIO Output 3" instead of "Focusrite USB ASIO Output 1" (which is for line 1/2) as your output in Sonar.
 
3) Line out 1/2 is used for the balanced 1/4" outputs, the unbalanced line out 1/2, and the headphone jack.  The headphone volume is controlled via the headphone knob, and the other 1/2 outputs are controlled by the Monitor knob.  They are all working for me.
 
4) I think the reason I wasn't hearing anything on line out 1/2 earlier is I didn't want to crank up the Monitor knob too much, so the signal was so faint that I couldn't hear it.

Although I don't have a headphone amp, I did find it interesting to know.
 
This might be helpful as well....I do have a first gen 2i4 which has been rock solid and just this past week (for the first time), it started dropping out audio/drivers at regular intervals (every few minutes). I went ahead and installed the 2nd Gen drivers and haven't had to look back since (although Focusrite recommends to use the 1st Gen drivers....I won't).
 

 
This looks like it's made to interface with a home stereo Hi Fi,, not a studio audio interface, So the weak signal is from a impedance miss match.
Did you try both the Hi and the Low setting?

Tripecac
I recently got a headphone amp (Schiit Vali 2).  For input it has phono jacks (red and white), which I connect to the Line Out 1 and 2 of the Scarlett.
 
No matter how high I turn the volume levels on the Scarlett or the amp, I'm not getting any signal on the headphones.
 

 
Tripecac
 
Line 1/2 on the Scarlet are for Monitoring via your powered monitors  Don't use this for anything else as this is the only out that you can adjust the output volume for.
 
For Output 3/4 on the Scarlet (Set Internally) this is at fixed level, so you would have to raise the volume using the bus Fader or the send level using either for a headphone mix.
 
From the Manual
 
The signals at outputs 3 – 4 phono (RCA) sockets are at (full) fixed level, and thus if you use these for monitoring, you will have to control the volume either at source (within the DAW itself), or by adjusting the volume on an external amplifier.
When you use Output 3/4 as a headphone source, make sure you change the front panel button for the headphone source to the 3/4 source next to the input for the headphone.
 
 =============================
I suggest using a Headphone Amp that has a single stereo input and multiple output with a volume control like the Presonus HP4.  Send the Scarlet 3/4 out to the input of the HP4 and you can adjust the volume from the front end of the headphone Amp for each person listen.
 

 
 
 
 

I was wondering why he bought that particular headphone amp? Seems sort of like you stick a tube on something and it might sound better??? I certainly don't want to colour my headphones thank you very much. 
What was wrong with the headphone output of the Scarlett? 
 
This is another point against the 2i2 and 2i4 is they don't come with Mix control. 
You really need mix control when you set headphone balance  up,, I can't see how it would work with out it?  

I bought the tube amp on the advice of people on an audiophile forum. 
 
My Sennheiser HD 580 headphones (which I use with Sonar) have been sounding a lot more trebly than my HD 590s (and various speakers).  This resulted in my mixes being too bass-heavy (as I was compensating for the lack of bass).
 
The consensus of the advisors was that the HD 580s were being under-driven and "need a good headphone amp".  When I asked which headphone amp to get, the audiophiles recommended Schiit, and the Vali 2 in particular.  As I know nothing about headphone amps, I wasn't one to argue.
 
When I listen to the HD 580s on the Vali 2, they still sound trebly.  I don't hear any tube-based "coloring" yet, but haven't done a careful A/B test.  I do remember that they seem to have sounded "better" when connected to the Scarlett's line out 3/4 than when they're connected to 1/2.  However, I would only get to hear the 3/4 output when using Sonar.
 
Do you not use a headphone amp?  Does the Scarlett act like a good headphone amp?  I'm still not sure what headphones amps do that's different from a standard audio card/interface.

The Mix control is basically done in Sonar depending on how many outs you have for your audio I/O.  you can us the sends on a channel to send to Out 3/4 for singer,  out to 5/6 for keys, out to 7/8 for drummer, and out to 9/10 for bassist.  what ever you want depending on your Outs.   If you only have a 34, out.  then you would send the entire mix to out 3/4 and route that to headphone amp
 
i have the Roland Octa-Capture, so I have 4 extra stereo out pairs that I can send to 3/4, 5/6, 7/8, 9/10 (Not including Out 1/2 ) which are for my Monitors. 

As I said the Scarlett 2i2 and 2i4 do not come with mix control so your faced with having to use the output busses in Sonar.
So I cannot say if the Scarlett 2i4 has a good headphone output as it will be different than my 6i6.
With the 6i6 I can easily use mix control and set the balance between the input and Sonars playback.
On my Tascam it is even better as there is a front panel control knob that does this.
 
That said I also use a small Mackie Mix 8 mixer to control headphone levels. For $60 it's a killer little mixer and it also allows for the addition of Reverb while tracking to the headphones. It is LOUD if you want it to be. It depends on what I'm doing or if other people are involved.
For you it would also give you a simple EQ. A mixer with more features would be even better. I go with that over a headphone amp any day as dollar for dollar a little mixer is certainly a lot more bang for the buck.