I have done my mod. AD8397 replaced with OPA1602. OPA1612 and OPA1602 sounded very similar and OPA1602 is even less power hungry. Output current is the same for both, 30mA but plenty for IEMs.
I used flux, a temperature controlled soldering station, desoldering wick, leaded solder, tweezers, a screwdriver and a light.
I ran into some problems.
First I was not carefull with desoldering/removing the AD8397 and loosend a connection pin on the circuit board. I put it back into place and started soldering the OPA1602 but again screwed up and this time the pin broke. I thought this is the end but I saw the pin was connected to one end of a SMD-part and figured I could connect the opamp pin to that. After soldering the pins on the other side of the opamp, I put on enough solder to connect the two points and made sure they were connected without making other shorts. All was into place, the HOLD-button was another obstacle.
Luckely it works and now it sounds a lot better to my ears. There is almost no difference between my iBasso D4 with OPA1612 and my X3, using T5 is no longer required to get the sound I want.
The mod is not easy! The most difficult part is removing the AD8397, if you do that in a careless way, you can screw up hard. I have quite good soldering skills but at my previous job I had a better soldering iron (tip).