Over 8dB is clipping level, and it looks like the digital FS level and max analog headroom are aligned at that point. That’s why it’s clipping at 0dB FS on the USB input.
Behringer specified THD+N at 4dBu (1,23V RMS), the pro standard pro level, and not at 14dBu (4V RMS) nor 22dBu:
View attachment 440768
If you want to keep THD in check, respect the 4dBu nominal level (maintain headroom, watch the VU meters, also when using the USB input). That’s how it’s done in a studio. The nominal level is the desired operational level. Higher than that is headroom where you can go on your own risk, usually bad practise. 4dBu output should be fine to drive active studio monitors which also adhere to this pro standard to max spl, most will be even more sensitive and provide a gain control.
Someone mentioned the Mackie Big Knob as an alternative. It also specifies THD at unity gain at 4dBu, with 22dBu as the clipping level. And the Focusrite Scarlett 2i2 reviewed a few months ago also showed degrading performance when driven to 0dB FS (although to a lesser extend).
That’s how this device is designed. As I said before, I also prefer a device with a lot of clean headroom, but I experienced poor headroom with budget pro audio equipment before (like with preamps of entry level digital mixing consoles). My conclusion is that with this interface $239 buys you lot of functionality, with mediocre THD+N and headroom.