There is no (read 0) correlation between ohms and sound quality, although some swear the beyer dynamic 600 ohm variants sounds better.
The FiiO would not suffice as it does not provide the voltage swing a 600 ohm headphone would need.
You can definitely find a headphone that sounds good out of macbook and ipod, but you should consider isolation too. Regardless of how good the headphone is, if you are in a noisy environment, it won't sound so good.
Rap and hip hop tend to be very poorly recorded and produced, so you should look more for a headphone that will complement those genres well over actual sound quality. D2000 and Ultrasone Pro 750 would be a good place to look. The audio technica suggestions weren't bad either. of the 3 brands mentioned, Denon is probably the only one that would improve with an amp.
I wonder if ohms and sound quality is more based on psi phenomena xD
There has to be an additional reason why phones sound better with greater ohms.