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The best noise removal software use a noise signature to remove the hiss or hum. So they require you to have a recording that starts with "silence". Actually it means there is no useful signal there, only the hiss of your recorder (that was the case with old analogue tapes) or only the hiss of your source.
They take the signature of that noise only part, so they can calculate the best way to remove it as effectively as possible.
The problem with noise (especially hiss) is that covers a big frequency range. You can't just remove hiss simply by eq'ing or simple subtraction, as you will affect the useful signal also.
So you must be sure you have the least possible noise on the recording you can manage to start with. Clean records, right recording levels, and quality/age of gear, etc are all very important in that.
As for the software, in my eyes Samplitude and Cooledit Pro have a more effective way of removing noise (better handling through signature noise). But nothing beats the Waves Restoration plugins (they have special algorithms to remove hiss, hum, pops and cracks, and clicks). Nothing beats the price either 
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