Nad M51 + usb conditioner?

Yep. I use AP2+PP to connect my linux nas to m51 and love the result. AP2 +PP sounds fresher while OR5 delivers denser textures. Both are great for m51 depends on personal taste and system.

Couldn't you just use the HDMI input from computer to NAD M51 instead of the USB?

I have one question here:  I have the NAD M51 in my system and use it as a preamp/dac. I'm also a computer scientist and have pretty good knowledge of how data is transferred between computers.  How exactly do you think an USB cable affects sound quality? 

Hi there, I’ve had the M51 a few years now, and still enjoy it.

I agree with the consensus - the USB input is not the best on the M51. What I recommend instead is actually the Yellowtec PUC2 Lite which is an excellent USB-AES converter. Gives a very meaty, powerful sound. I prefer it to the iFi iLink (USB-SPDIF) which is laying around unused.

From there, you can add as many USB clean-up devices as you want. There are tons of (too many) these nowadays (Uptone, Intona, new iFi products). I still use the old iUSB and Mercury and Gemini cables.