I assume you're using the USB as an audio input, so just to make sure, it's showing up as an audio device in the Studio One audio set up, and you've got it showing up on your track on the input device right? (white arrows), and, you have the track armed for record and active. (red arrows). If that's the case, is there a way of increasing the output from the amp?
VST and rewire support won't help with this since it's an audio device.