Fixing that Low Volume Headphone Issue
There are two things to try:
First- make sure you’re recording with a solid enough gain. Often people complain about their headphone volume being too quiet, but they’re actually recording at super low levels. If you’re doing that on purpose to magically remove noise from the background- stop. It doesn’t work. Turn that knob up to a respectable level.
Next, don’t be shy about cranking the headphone volume. Maybe like all the way, as seems to be the case with the DT770 80Ohm paired with the Scarlett Solo.
If neither of these things is your issue, than I suggest a simple headphone amplifier (unless you’re planning on upgrading your interface). The Behringer Microamp HA440 is a compact unit that will even let your plug in 4 pairs of headphone, should you ever need that. Oddly enough, at $23.99, it's still cheaper and easier to connect than most single headphone options.