I bought a new subwoofer and there are a few switches at the back of it, one of which I am unsure which setting it should be on.

There is a switch and you can put in on either NOM or REV.

Anyone know what the recommended setting should be?

What make/model?

It's a Yamaha SUB

The Nom and Rev switch is for changing the phase of the speaker in the sub to help match it to the other speakers in your system. There is no real recommended setting, basically set it to the position which produces the best sounding bass to your ears, one position will sound like it's producing more bass than the other, the one that produces more bass will be the correct setting for your system.

Thanks.

The REV setting seems to give a fuller base and be more in sync with the speakers, so I'll leave it on that one.