Ayre QX-5 vs Lumin vs dCS Bartok

I have owned the Ayre QX5 and the Lumin X1, but have not heard the Bartok. I owned the AX5 Twenty when I had the QX5, but have now moved on to something else, although the AX5 Twenty is a great amp.
Both the QX5 and Lumin are great dac/streamers, and I doubt you would be unhappy with either. I preferred both to the Chord DAVE/M Scaler I owned previously.
The QX5 has a big, detailed, powerful and muscular sound. It can sound very slightly dry (as can the DAVE) and has a tonal balance just a shade lighter than neutral, but I think this is just consistent with the Ayre sound, since the AX5 has similar characteristics. It is very engaging and I often found listening to it that I had a real sense of the recording acoustic and the presence of the musicians.
But ultimately I preferred the Lumin X1. It has a bit less of the "master tape" realism but is tonally a bit warmer and more colourful, and has a more liquid, saturated sound.

It's really down to personal preference. If you like the Ayre "house sound" and prefer a big, airy soundstage rather than a warmer, slightly darker presentation, you may prefer the QX5. The QX5 is also a great match with the AX5. I preferred the tonally richer and more liquid sound of the Lumin, but sometimes still miss the QX5.

Put MSB Discrete DAC in your list. It has MSRP right under $10K. The second optional power supply is $1,500. The Renderer module about $1,500, and the ProUSB ProISL module combo about $2,000.

I can put in a good word for the Ayre.  I owned the QX-5 Twenty for a while and really enjoyed it.  I found it to be one of those "just works" pieces that never gave me any trouble and always sounded great. I used the network input as a Roon endpoint, the balanced headphone outputs with my Focal Utopia, and the balanced preamp outputs with my Genelec 8030C monitors.

Surprised this thread didn't go further.  I own the AX5/20 (selling as I have the Vandersteen HP5 amps in bound).  I owned the QX5/20 and LOVED it for so many reasons.  Just a great digital hub that sounds great.  The Lumin is also a great DAC and I agree with the first posters answer in the differences. I personally like how 'fast' the Ayre gear is.  It is so revealing.  Whenever I change out and try something else, I realize that over the long haul, I love how detailed and still musical the Ayre gear is.  Personal choice and I'd be happy with either.  

I have auditioned the Bartok on many occasions and feel it also is a nice DAC and worth the price of admission.  I don't feel it's better than the QX5/20 though.  

I own the QX5/20 and have used it for a while as a Roon endpoint on my network with an i5 core in another room.  Very nice sound much like @rossb describes.  Recently added a Wolf Alpha 3SX server to my rack and connected it by USB through the Wolf’s reclocking USB output.  I did not know the Ayre QX5/20 could perform at this level...  very rich tone now as well as the previously noted air and detail.  Much more muscular in presentation.