I'm using an amp that is compatible with my earphone. The problem is its a analogue controlled amplifier that has channel imbalance issues at low volumes. On top of that, having a sensitive iem doesn't help. The amp works perfect with line out attached to an hd 650 though.
The other solution is to buy an amp with digital volume control that has no channel imbalance issues.
The solution I'm looking into right now is to buy a new amp with modded negative gain to help with sensitive gear.
I'm willing to bet if you were to put into your amp a tone of say 15K Hz and you hooked up an oscilloscope to both out puts you'd see that at low level output the left/right signals are not matched as to voltage level output... this is either a design flaw or most likely a mechanical problem with your volume potentiometer... ( you did say it was analog ) One thing you can try out before you decide to get a new amp is to use a potentiometer cleaner called DeoxIT. You just put some on the pot so the liquid goes into the body and de-oxidizes the wiper mech. Use it all the time on my Peavy AMPs... just nature of the beast..
Another solution for you is to get a couple of in line attenuators and put them in between the amp output and the sensative earphones...that way you have to drive the amp out of the channel imbalance region... I don't think the X3 is the issue, but it might be bringing the amp problems to the surface a little more than your used to.. The reason it works better with your HD650 is you are turning the volume knob up to a different position... probably past the oxidized part of the wiper mech of the potentiometer. Or if its an older amp.. you might be experiencing component fatigue... caps starting to leak ???