I’m positive you should get a solid sound on your focusrite. That is the whole reason why you have a DI out in your amp so that you can plug it in your audio interface. 
The best thing to do is eliminate every option until you find the culprit. Personally I think it could be something with your cable (for example is it TRS and not TS). I did read somewhere that the headphone output on the scarlett is a bit weak, but not as extreme in your situation.
So here goes:
- Is it the right cable? Is it plugged in correctly? Maybe similar to what is said in this post: https://www.reddit.com/r/Focusrite/comments/cxocpb/focusrite_2i2_3rd_gen_very_low_volume/
- Does your audio interface needs a firmware update?
- Has your computer/DAW done something to your input and output settings of your DAI?
Also I read this somewhere: "On the scarlett there are two volume control knobs on the front of the interface. 1 controls the headphones and 1 controls the monitors. Get the balance between your mic input and instrumental correct in your DAW and then control the volume as suitable for heaphones/monitors. "
Hope this helps. I would do some investigation on the cables and your computer settings first. Good luck! 