Agree with the quality of amps used with the NS5000. The NAD is not in the best choice. I would take a Benchmark AHB2 mono over the NAD for about the same price, even though it was my worst performer on the NS5000.
With regards to the current or power of an amp I have to disagree on what is needed. I just finished a shootout with 3 amps,
- stereo Benchmark AHB2 (100 | 175 | 192 @ 8 | 4 | 2) (off memory)
- CODA #16 (150 | 300 | 600 @ 8 | 4 | 2)
- Sanders Magtech (500 | 900 @ 8 | 4)
All 3 of these amps drove the speakers to acceptable levels. The bass was incredibly articulate on the AHB2. The AHB2 is the weakest power wise of these 3 amps and the CODA is the strongest. One could argue the bass was the best with the AHB2. My friend, who dislikes the AHB2, even said this to me on the Stones Sympathy for the Devil track. My takeaway is that the NS5000 is not that difficult to drive even though it has that huge woofer.
The AHB2 sadly came out last in the overall test, I wanted it to win. The AHB2 had a slightly smaller soundstage and some fatigue in the top and mids with my gear. I am very sensitive to this, and others may not notice. I may be able to eliminate that with a warmer DAC, but the issue was not there with the other 2 amps.
Now if you want to stay in a "budget" price range the Sanders Magtech is an excellent choice for the NS5000. My used Sanders was $4k from Sanders with warranty and is excellent in many ways. Now if I had not heard the #16 on the NS5000 I could be very happy long term with the Sanders. The Sanders is miles ahead of NAD M22 V2 that I had in the house long ago.
I also tried my modded Peachtree GAN1 (Class D 200 @ 8) and it was too weak to drive the NS5000 nicely. Though I will try this again as a stopgap tomorrow. I may sell the Sanders and AHB2 to raise cash for a more expensive amp.
Given my taste for a neutral sounding amp fo the NS5000, that suggestion of the Boulder amp is interesting. The Diablo would be too warm for me. A SImAudio North Collection is what I think would work the best for my ears, but I have only heard the old lineup. My CODA #16 is an amazing amp, but I like it better on my Magnepan LRS+ speaker (spectacular pairing with Holo Serene preamp + LRS+). The LRS+ is nowhere comparable to the NS5000 but the #16 takes it to another level of goodness. The Sanders comes pretty close to the #16 on the NS5000.