I have several older Shure SC4 wireless receivers, which have a Mic/Line switch on the back. Last night I need to use 5 of them with my UR44C. I set one of them to Line and plugged it into Line In 5. I use a standard XLR female to Male TRS line cable.
Initially all I heard in my headphones was a very low level with heavy echo. I opened the dspMixFx UR-C on my laptop, and changed the settings to match the defaults for inputs 1 through 4, but increasing all of the levels and making sure no effects were turned on. I was able to get a usable level from the channel, but there was still a huge echo/reverb on the channel.
I tried Line In 6 and I tried multiple SC4s with different microphones.