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.
Connecting this headphone amp is easy- but you need to have the correct cable! A stereo 1/4" cable will connect from your interface headphone out to the input of the amplifier. While the pictured Hosa cable lets you choose the length you want, you only need enough to connect the amp, since you want to keep your interface inside your booth/ closet/ teepee/ vocal wigwam/ acoustically treated thatched hut. Whatever brand cable you get to make this connection- just make sure it's "TRS" (tip ring sleeve) style. You can tell because the tip has 2 black bands around it and not just one. If you try to connect using your friends guitar cord that's hanging around- you might end up with mono or everything in one ear. Just make sure you have the right thing!
I hope this helps.