...but what happens when you connect the sub directly to the NAD's LFE outs? It's possible that if you have the NAD set to "sub = yes" or whatever, it's trying to send any true LFE info out the LFE output which, as per your description, goes nowhere.

I know the early NAD DD receivers have some bass management issues with that series. Still, it may be better than what's happenin' now.

Also, when using a receiver's pre out/main in loop, you must set "sub = no" and "main speakers = large". The LFE will be directed to the main r/l channels instead of the LFE out.

Likewise, if all else fails, many subs have speaker level inputs which work quite well.

Lemme know if this helps.