Ohm alone, on the phones.

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.

Curses!  Foiled again!

Thanks.

You just need more gain. Something like a simple amp. You don't even need the volume control since you're controlling that elsewhere. There's no shortage of reasonably priced such amps. And there's much more expensive options like driving them with a First Watt amp right off the output posts.