Offically, I have to tell you that N5ii is not certified for aptX and probably will not be in the rest of our product life, this is what we have to stand by. Promoting aptX compatible required a long certification process, and will affect our existing BT license such as BQB (Bluetooth Qualification Process). Revising all our Bluetooth license worldwide will cost a hefty amount.
Off the chart, I'll suggest you to find an aptX bluetooth headset and try it out for your personal exploration. That's the best I can say.