A sanity check would be to try the M50 headphones in a headphone out that is known to have more than enough juice and power, maybe a Mackie mixer or anything else you can find that has a powerful headphone out. If the M50 doesn't distort then you know it is the EMU and MacBook amps that are the problem and not the headphone.
The M50 has a relatively low impedance (38 ohms) and a lowish sensitivity (99 dB). That combination means the amp has to supply more current. Some amps just can't supply the necessary current for low impedance headphones and when they can't they distort. The same amp may do fine with higher impedance headphones.
I can get my M-Audio FireWire 410 headphone out to distort if I use a low impedance and low efficiency headphone. The headphone out is just wimpy. I suspect the EMU headphone out is similar. Laptop headphone outs are known to be wimpy as well.
One solution is to get a suitable amp that can drive the M50 properly. Otherwise look for a higher impedance headphone with higher efficiency that may work better with the EMU and MacBook.











