I am having the same problem. I’m not right by my equipment, so I haven’t tried line in the back. Have UR22 and Jamulus combo, so it’s a little complex.
Today I’m reading forums making a list of all the things to try and now plugging in the back is at the top of my list (not ideal since it would be nice to control the volume).
Since setting up last weekend, I have had headphones in the computer jack in order to move ahead. But that’s not ideal.
In addition to tinny sound overall (Jamulus listeners hear it), when I plug the headphones into the AI front headphone jack there is also a loop coming in from the system. I’m trying to use this for Jamulus, and so the monitor gets turned down (i.e., 100% DAW signal coming from the server), but no matter what, the inputs are picking up the output server signal. Other people say they hear an overall echo, only when I’m on.
I’m sure it’s something related to me not understanding sound engineering (balanced cables? shielded headphones? USB 2 vs. 3? need a power supply for UR22?). The setup is UR22 mkII vocal in left channel via XLR, guitar in right channel via line in, USB cable to Thunderbolt, Ethernet connection to Jamulus server. Basic low-price studio monitor headphones, with dual plug if that makes a difference, 3.5 with a screw-on line jack.
The echo goes away with the headphones plugged into the computer jack . The tinny sound does not. I’m hoping someone with more sound engineering experience can suggest options because my to-do list is about 30 items long and I’ve already spent 20 hours on this!