To answer jmtennapel’s question, and I might be wrong, but I think that the ‘subwoofer’ output on most Naim equipment is actually full range and therefore really just a pre-amp out. If so, given that the NAD has high pass filtering on its subwoofer out, the OP is looking to limit the lower frequencies going to the main speakers by application of said filter. The problem will arise when the OP wants to adjust the volume as both the NAD and the Naim will have different gain structures etc.
Regarding Naim and HPF on the sub out, I am aware that the UnitiQute and the Uniti had it, but the Unitilite and Superuniti didn’t - I have no idea why this was the case. On the units mentioned that have it, I think it was set at 100hz (possibly higher) which for many larger speakers is simply too high in my book. As mentioned before, perhaps someone with detailed knowledge of Naim preamps can tell us if any equipped with a subwoofer out actually have any form of HPF. If they have, is the frequency adjustable?
Interesting comment about subwoofer positioning made above. After a lot of research I have found that the large (15 inch driver) Tannoy subwoofer I use in my main system has finally integrated properly when repositioned from on-axis between the speakers on the same wall to halfway between my main speakers and my listening position along the side wall. The sub ‘fires’ across the room from the left and ‘intercepts’ the main speaker output (large floorstanders running full range) about where my feet are. The continuously variable phase is set at about 15-20 degrees. Doing this has made a considerable difference.
In car audio circle HPF is common place - I use it on the fully active system in my MGB. How you set it depends upon which crossover slope you use as it can result in a boost, cancellation or smooth frequency transition depending on the slope - a useful video on YouTube explained it all. The head unit in the car has an extensive range of DSP and EQ adjustments including adjustable crossover slopes - 6db/12bd and 18db - I think I went 18db in the end to avoid cancellations. The unit also allows programming of the exact distance of each speaker from the listening position, the size of each speaker, the type of vehicle and stage height to be adjusted electronically.
The office system I have runs a BK Gemini II and the Mission speakers are full range, with no adverse effects - the speakers have never shown signs of suffering. Conversely the AV system is set up with an HPF as the glass Celestions I use as stereo fronts really benefitted from having an HPF applied by the AV amp and the lower frequencies routed to the Active sub.
Finally, I do have an Arcam Solo Movie 2.1 that has the ability to set an adjustable HPF/LPF crossover point. Ironically I do not run a sub in that system but I keep meaning to engage the HPF to see if it will make any difference to the performance of the little Piega speakers that I use with it. It would mean that it would divert the lowest frequencies to a non-existent sub but it could be worth trying.
Overall, smaller speakers may well benefit from HPF but it needs to be properly implemented, otherwise run full range on the stereo pair and take care to position/set up the subwoofer properly. Obviously, if you hear the mains ‘strain’ at very low frequencies then HPF is a good idea but I would be surprised if most HiFi speakers really need it.