[simon.a.billington]

If you're looking at just adding a sub octave Waves Submarine is the most natural sounding sub plugin that I'm aware of.

If you want to drop the entire pitch by an octave that gets a bit tricker as it is hard to get it sounding natural. RX is a good option, but pricey and not a good option if you want to do it in real time.

You could try looking at Nectar or their VocalSynth plugin, but the latter is most likely to sound less natural. Or there is WavesTune RT, not sure if that can drop sounds an octave, though. Then there is Ovox, but like VocalSynth there's a good chance you may not get natural results from it.