Your problem is probably jitter. Since you are using S/PDIF coax from the Bluesound to the DAC, there is an opportunity to minimize this. Not that the NAD DAC is perfect, but It will probably be a LOT better fed with a low jitter source.
On the other hand, everything that the NAD gets is evidently resampled and changed to PWM signal, so it may not be a huge improvement.
You might try USB first. You will need a good USB cable from Bluesound to the NAD. I personally use the Wireworld Platinum. Also, it is a good idea to use this in the USB cable:
https://sotm-usa.com/collections/sotm-ultra/products/copy-of-tx-usbultra-regenerator-1
This regenerator will improve things a lot with USB, particularly bass focus.
If you want to try reducing the jitter from the Bluesound S/PDIF output to see if that helps, you could try one of these:
https://www.audiocircle.com/index.php?topic=157348.0
7 psec of jitter and 30-day money-back, less shipping. I highly recommend getting a better S/PDIF coax cable, BNC type with adapters for RCA.
Steve N.
Empirical Audio