High ohms require amps with high voltage swing while low ohms require more currents.
Volume wise, low ohms are easier to drive.

It depends, say akg k701 and th900, both are low impedance but the akg are so much harder to drive to get the optimal sound quality whereas the th900 sounds good even out of an iPhone.
If your purpose for amping is mainly for volume, my iPhone can drive my HD800 to an acceptable level but sound quality was horrible.
So my suggestion is go and audition first to see your requirements, wether an amp give you that improvement that you desire.