I use U18t with bal out of SPKM (1ohm) and the sound is ok. 2ohm SE output of SPKM is OK too, there I use U18t with 3.5mm adopter. In general, U18t is OK with outputs up to 2ohm, but going higher will cause issues. I covered that in my recent review of l&p L6 where se 2.2ohm output was OK, but bal 4.4ohm messed up the sound.
My current companion to my SP1000M is LCDi4 and there is an EE Legend-X in the post hopefully arriving next week to cover the cases where I need full isolation. They are both SE connectors (LX is second hand, so I didn't have a chance to choose). The question is, do you think it would make sense to go balanced for these IEMs? I would not expect the LX to go up to 300 ohms and I already like the sound coming out of i4 especially in the bass region, but at least I can modify the cable of LX myself and maybe buy a balanced cable for i4, if it would really be an upgrade. What do you think?