Best DAC

It depends on your budget but for pure delta sigma dac go with (as someone recommended) rme adi-2 (be sure is the Rev 2),if you can stretch some more holo audio spring 2 is an excellent R2R ladder dac

After that you have Holo May and Denafrips Terminator

A few thoughts: if you have the space; you might consider using  some of the funds towards bigger maggies. Also, when was the Threshold S500 last recapped? Depending on the DAC you choose; you might find the SP6 superfluous.

Definitely time for a DAC upgrade.  An Audio Mirror Tubadour Mklll SE would be a big upgrade across the board, and they offer a trial period so you can try it in your system (which is obviously huge).  Same for MHDT Pagoda if you run balanced.  Agree with others to use some form of a noise reduction device if you continue using your computer as a source.  Best of luck in your search. 

the arc sp-6 is a fairly honey colored unit (a good thing) but with decent high end extension and an expansive image (i used to have one many moons ago) - you did not mention which version you have... of course you can roll the 12ax7’s or 6dj8’s inside to shift more or less honey colored

of the dacs mentioned so far which i have tried i would break into the following groupings - for your benefit in system matching for tonality

smoothest/most analog/warmest mids-midbass: mhdt, audio mirror, a-n - bear in mind these dacs can have tonality tuned with tube choice as well

slightly brighter: denafrips, schiit mb (gumby and yggy)

brightest/leanest/’fastest’: rme adi2, mytek bb

others to consider which are exceptionally natural sounding and great sq for $ -- metrum and ayre codex (fitting in 1st group), ava valve dacs (second group)