The first thing I would do is check to see if the signal is actually getting from the amp to Studio One. With the console view open click on Inputs to the very left. Look for the signal coming in there. The manual says the effected sound is being sent on Windows Primary. On Mac Stereo Out 1/2.

Is the signal arriving there. From the manual it looks like it is being sent all the time. If the signal is arriving there then the track needs to be Rec armed and input monitoring selected before you will see it and hear it monitoring through Studio One.

I guess you could monitor the guitar sound via the amps speaker. Let us know how you go.

Looking at the block diagram for analog recording I would be using the LINE OUT (not the REC OUT) The REC OUT comes after the Master stage whereas the LINE OUT is at the same point as where the USB signal is being split off. (Before the Master stage) Whatever the Master stage actually means. I would imagine the LINE OUT signal to be of very similar quality to the USB feed.