I wouldn't automatically assume that 'more watts = better quality'.
Trailer "more watts = better quality" is nonsense."
It depends on what you mean by 'quality'. More watts certainly doesn't mean better quality equipment. Just the opposite, for a given price, a lot more watts probably means poorer quality equipment.
However, all other things being reasonable, more watts does mean better quality sound in the sense of more clarity and dynamic range.
In general, within any given brand, moving up to a more powerful model usually means better quality as well as more power, both of which are to your advantage.
But, as most people mistakenly assume, more watt does not equal more sound; 'more sound' meaning louder. It takes a huge change in power to increase the overall loudness by a noticeable amount.
What more power gives you is usually proportionally better overall equipment quality, which is why the price of an amp rises so quick as the power rises, and more head room.
Head room, like perceived loudness is not linear. Twice the voltage signal head room, mean four times the power to accomplish it. But four times the power, is really only +6db louder, or in layman's terms, noticeably but only slightly louder.
So a step up from a 50w NAD to an 80w NAD is indeed a step upward in both equipment and sound quality.
Again, in this particular situation, I would be hard pressed to choose which to upgrade, my amp or my speakers. Though, I do think a 50w is enough to get the job done for home listening. As a side note, my current amp is 55w/ch.
So, I would be leaning toward some bigger and better speakers. For me, it would likely be a pair of Diamond 9.6 or possibly Monitor Audio Bronze BR5. But if this person is reasonably happy with their B&W 601, them perhaps an upgrade to B&W 685 might be worth considering.
Again, because this is such a reasonable well balance system to start with, I would be torn which direction to choose. It would get down to BUDGET and impulse.