MR816x inserts: balanced source output to unbalanced input


I have a GSP1101 and I want to connect it to my MR816X inserts, bypassing the preamps.
Since the line out source on my GSP1101 is balanced, the guy in the shop told me to get a TRS cable.
Now the inserts on the MR816X are unbalanced, so when I connect the GSP to the insert, should I only half insert it (ie, to the first ‘click’)?

I tried to fully insert it and noticed a drop in volume and a loss of bass (impedance mismatch I’m guessing). Half way in sounded fine, but I’m wondering if I need to use an unbalanced cable end for the 816 insert? I’ve no reference and I dont want to buy a TS cable if its going to result in the same sound.

Does half-in on the 816 insert with a TRS cable equal the same as full in with a TS cable?