It can only make it worse IMHO. If the hiss comes from the output stages then lower ohm ratings will make it worse. Hiss coming from the input stages will appear the same, since volume control won't change signal to noise ratio.
Ohm alone, on the phones.
My headphones have an ohm rating of 64. I am getting a tiny bit of hiss from having to turn my headphone feed up so high. If I reverted to a set of cans with a lower ohm rating, would they seem louder, and therefore, I could turn the headphone feed down.....and get less hiss?