I don't think he has a choice.
Unless I'm mistaken, the NAD only outputs a full range line level audio signal. The HSU x-over function will be necessary to route the frequencies appropriately. He may have a choice whether or not to run the Wharfedales full-range (I'm unsure if the HSU outputs a crossed high pass signal to return to the amp for feeding the Wharfedales), but - either way - the bass management will be performed at the sub.
He may also connect at speaker level, but - again - the HSU will provide the bass management there.
BTW, please cheerfully ignore this post if I've misunderstood the question or don't have the NAD's capabilities right.