I presume you are referring to Line Inputs 5+6 on the rear of the UR44?
First up, have you swapped out the lead from iPhone/PC (presumably a stereo mini jack to 2 mono 1/4" jacks) to check it’s not the lead?
If the lead is ok, is the poor sound only from the a headphone output (headphone output 1, 2 or both) or is it also from the main outputs of the UR44?
I have a UR44 so I could try and replicate the problem if you provide specifics about cabling, set up etc.
A quick follow up. I have a Roland Fantom G plugged into Line Inputs 5+6 and when I have the UR44 connected by USB to my laptop, I get a horrible digital background noise. The noise gets worse during disk access and screen updates.
However, replacing the mono (unbalanced) 1/4" leads with stereo (balanced) leads eliminates the noise issue.
I then tested the output from both my Samsung S7 phone and iPad2 using a mini stereo jack that splits to two 1/4" mono jacks into inputs 5+6 of the UR44.
The sound from both the main and headphone outputs were clean, clear and as expected.
I did have a faulty unit from new (as were many due to bad components) and my UR44 was from the first batch released, having been on pre order 3 months before release.
The fault was hissing, crackling and a very low input level on channels 1+2 but I would have thought that all new(er) UR44 units would not contain the problematic components. Still, worth a mention.
Would be good to hear if you resolve your problem.