If refined sound is what you are after, I would look somewhere other than that NAD amp on the tweeter. Personally, I would use one NAD amp on the bass driver and a good moderately powered tube amp up top.
Exactly. Check my posts you will see that the plan is to use the NAD 218 for the bass. This would just be an experiment. My current integrated amp, Ragnarok, sounds very good in mid/treble which is why my NAD separates are idle. But using the NAD for bass with speakers that are biampable may be worth trying. And I am thinking just like you are thinking “I would use one NAD amp on the bass driver and a good moderately powered tube amp up top”. Perhaps Raven or LTA as an upgrade down the road. That is one thing I find appealing about the X3 is amp flexibility but can’t afford those right now so I ordered the new M3. That said I have a strong feeling that I will be very pleased with the M3 Sapphire biamped or not. And if I had to guess now, the Ragnarok integrated by itself may sound better than biamping. But would be interesting to try biamping anyway since powerful NADs are sitting there idle.
P.S. My Dad upgraded his NAD 218 to a Musical Fidelity Nuvista KW500 amp several years ago. He then gave me his NAD 218. Since I already had a NAD 218 I ended up with two. I got a Schiit Ragnarok that I was strictly going to use as a headphone amp but tried it with my speakers to satisfy curiosity. I really did not expect the Schiit Ragnarok to out perform my NAD separates but it certainly does. The one exception would be bass power of the NAD. Ended up being a blessing in disguise because now I use my best sources for both speakers and my high end headphones through the Ragnarok.