This can be easy to overlook and in fact in my first version of the studio builder I chose not to include it.
However, I came to find in my own experience that this is actually a very important piece so hopefully I can help you avoid that pitfall in your studio.
To use your Audio Interface properly, you need a good, high output signal to your headphones so that you can set your input levels correctly.
In other words, you need to hear the signal loud enough so that you can make intelligent decisions about how the levels need to be set.
There are 2 types to choose from and you can get the full story at the
Headphone Amps Page
where both the Distribution and Sub-Mixing varieties are compared.
Basic Studio Builder Rating - $
You don't need to spend much, but a dedicated headphone amp will make your life much easier. If you can't hear what you're doing then you may as well just pack it up.