I’ve just checked the USB level indicators using a test oscillator. The meters are green up to -10dBFS (digital zero), then a small yellow sector between -10 and -8 dBFS. Between -8 and 0dBFS the meters are red, and bottoms out at 0dBs. The QC does not clip or compress anything below 0dBS.
So also in USB there is room to boost the output signal (using the output block gain or any other module with gain). I’ve noticed most factory presets delivers a peak of about -6dBFS and in the DAW (Logic) I have added a +6dB gain module on the QC input channel to match the level when playing to other sources.
As for studio use, NDSPs decision to calibrate the gren/red/yellow at a “pro” level of their meters, about -8 dBFS, is a good choice IMHO. It’s the same with their plugins actually, which require about the same boost to “fill up” all headroom to get close to 0dBFS.