Bluesound Vault 2 vs NAD Masters Series M502

OP
the two products that you are looking at are both made by the same manufacturer—NAD.  I have the Vault2 and am not familiar with the M502 but I believe that all NAD servers use the same control app, namely the Blue OS.
i would have a hard time believing that the M502 is worth 4 times the cost of the Vault2.
   The DAC in the Vault2 isn’t anything special, and I have always used it with external DACs.  If you have a DAC that you like, save the money  that you would be blowing on the M502.  If you don’t have a DAC, I would still get the Vault2, spend perhaps $2K on a really good DAC, and still have an extra grand to spend on Music., or perhaps a Roon subscription.
  My experience with servers, computer based or otherwise, shows that the quality of the the playback experience is largely determined by the quality of the playback app.  Since the 2 players that you are looking at use the same app, I would go for the cheaper product

You might consider separating the functions and widen your possibilities. For example, if you use a computer to rip the CDs, then you don’t need a server with the CD drive, opening the door to many more systems.

I’ve gone through various incarnations of my digital music setup, but my current one I like the best. I use an Intel Nuc i7 running Roon Rock software. $500 for lifetime subscription to Roon, and about $600 for the Intel Nuc. This feeds my dac via Ethernet. Roon is amazing audio software, You can control it from any smart phone or tablet or any computer.

If this sounds interesting to you, all you have to do is find a DAC with an Ethernet option, and you’re set. PS Audio has a few great options in the gear that uses the bridge II. Other DACs also have an Ethernet option, but in fact they are in the minority. While you can use USB, I feel that ethernet is far superior if implemented correctly. My experience with PS audio’s implementation was excellent and my current experience with Devialet is also excellent regarding Ethernet for digital. I’m guessing that these are not your only two options!

It’s true that having the cd  ripper built into the machine is convenient, but after you rip your entire collection, most likely you’re going to move away from CDs to some degree if not totally. And even if you continue to acquire CDs, you can still rip them with your computer and simply transfer the files.

If you’re not familiar with this type of approach, it may seem slightly daunting. It really isn’t. Simply a process that once you begin, it becomes super simple.

I know this isn’t exactly what you asked, but I hope you find it insightful!