You canāt, or should I say shouldnāt, try to connect a balanced output to a single ended pair of headphones. You can go the other way and connect a single ended output to balanced headphones.
The NDH30 are internally balanced, but the cable is singled ended. If you buy (for example) a 3.5mm female to 4.4mm male adapter, you risk damaging the amp. I hope I have understood you correctly.
FWIW there is no such thing as ādegradedā single ended as output vs balanced. They are just different topologies, but it just so happens that balanced puts out more power.
The Neumanns will run quite happily single-ended so long as the amp has enough power, and I donāt know your ifi device, but I would image it has plenty enough juice single ended for you to be able to make an informed decision on them. The NHD30 are not particularly hard to drive.