What would be an upgrade over the NAD M51 Direct Digital D/A Converter?

jb:  I use a Cary DAC200ts in one setup.  Just using it for CD and Tidal, for now.  I think it sounds pretty rich and engaging, with upsampling and tube output.  Although lose some detail that way.  If that's not where you want to go, might not be that much of an upgrade over the NAD, if at all.  May be even a step backwards, depending on your preferences.  Probably could find one used eventually for around $2k-ish.

A Schiit and a NAD M51 are on my short list whenever I get around to upgrading a different DAC.  So, for selfish reasons, I'd recommend you try the Schiit and report back on your thoughts on how it compares to the NAD.  :)

You know you're right. It may be tough to beat this DAC without dropping a fortune. I may just need to realize that my analog front end is just better and my digital is very listenable just not going to be as good. 

Try a stand alone preamplifier. after trying the Auralic Taurus pre i couldnt go back to using the M51 as a preamplifier

I upgraded from an M51 to a PS Audio DirectStream DAC when it first became available and haven't looked back.
The DirectStream DAC does not use a conventional DAC chipset, instead uses an FPGA, so once or twice a year PS Audio makes firmware updates, once installed, improves the performance of the DAC, something that is not typically possible with a DAC with a conventional DAC chipset.
No plans to replace the DAC, from someone who tries stuff out and does lots of upgrades.