Focusrite Scarlett 2i4 - How to send line out 1 and 2 to headphone amp?

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.

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).
 

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.
 
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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 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.