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.
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.