Adding a tube DAC to my BLUESOUND VAULT

I recently purchased a used BLUESOUND VAULT ( cd ripper and streamer) and after reading a lot of posts on various sites I may add a Tube DAC in the $500 or less price range to my setup  I have the VAULT hard wired via CAT 6 > RCA out to RCA INPUTS MFA MAGUS A2 tube preamplifier > MEITNER MTR101 MONOBLOCKS > BOZAK URBAN speakers. So, thank all in advance May I have some ideas and suggestions please?
I think you may need to up your budget to the $1000 or so level and look for a used MHDT Orchid or Audio Mirror Tubadour.  You might also consider a solid state R2R NOS DAC that tend to have a more “analog” type sound, but those too will be tough to find at $500 or less.  Best of luck. 
$500 for a tube DAC, I’d go way back and find a California Audio Labs Alpha if you can find one. That was a nice tube DAC in the day and I bet will still hold it’s own today. The CAL Sigma was the model below and still decent but not as good as the Alpha. 
Other option and new would be the Jolida Glass FX Tube DAC II. I’ve not heard one but Jolida’s always get good reviews. 
I have a somewhat similar system, tube preamp, Meitner MTR 101s recently updated by John Wright and a Bluesound node 2i.  I am using the DAC within the 2i, but what made a nice and noticeable difference was going from the ethernet cable that came with the unit to a Supra CAT8+ purchased from zendada on Ebay for about $50 including shipping. He calls it CAT8+ but it really is CAT8.  Not sure how much, if any difference it would be going to it from your CAT6.

If you do go this route be sure to purchase it from zendada, the other ebay seller is in England so shipping is another $20 and takes three or four weeks to get here.

Besides the Supra CAT8 cable if you chose to, I wonder if you need new tubes for your preamp instead of a tube DAC.  The MTR 101s are somewhat "tube like" and you already have your tube preamp. I only mention this as I had a substantial and surprising improvement when I changed my preamp tubes, the old ones lose their sparkle so slowly you don't notice it until you change them. 
Thank you all. I am just now starting my search and may have to consider SS to stay on budget.