Using Eleven Rack, Audiobox USB and Studio One 3

Hey guys. First, I must admit that I am just starting to learn the ins and outs of home recording hardware set up. I like to play and record, but am a bit of a noob when it comes to the technical aspects. I'm ruing Windows 10, Studio One 3 Artist and the interface I've been using is the Presonus Audiobox USB https://www.presonus.com/products/audiobox-usb . It came with the DAW.

So, I just got an Eleven Rack. My main question is: how would I use the 11r with the Audiobox USB? I see one or two other sort-of related topics around the web, and people are talking about coecting the 11r via XLR outs to the interface, but the Audiobox USB doesn't have inputs for that. Right now, I have a cable ruing from the "Output to AMP" out on the front of the 11r into the front of the Audiobox, and I am getting a strong recording level, but can barely hear the guitar coming through my headphones. I've tried messing with the Audiobox's knobs (Mixer, Phones, Main), but nothing helps. The instrument/mic input knobs are set just under the clipping level.

Anyone have any ideas or suggestions? Do I absolutely need to get a new interface (that has XLR ins) to run the 11r through? Don't really have the money to spend right now, so I'd like to make it work as-is for now.

Any help will be greatly appreciated. Thanks in advance!