Do the USB-SPDIF + DAC count too?
If yes, the Concero + Metrum Quad is, to my ears, better than the Concero in standalone.
My ranking (quick):
#1 - Audio-GD NFB-27: Has it all (details/definition, separation, authority/impact, big soundstage, organic tone) and offers something really special: excellent extension in the treble without a hint of harshness! It features an excellent amp and preamp too.
#2 - Concero + Metrum Quad: Less technical but has a different presentation that is equally enjoyable (crystal clear, natural tone of R2R DACs). The technicalities are that of the Concero, actually. It even gets its sound signature (that's how transparent the Metrum DACs are) but removes the (slight) hardness and adds the R2R magic.
#3 - Concero standalone: For the size, a killer DAC. It's less refined than the NFB-27, brighter and harder sounding (w/ the HE-5LE + SA-31) but it will fit in your back pocket (don't sit on it, you'd break your pocket!) when the NFB-27 needs a truck.
#4 - Stello U3 + Metrum Quad: Warmer and more romantic, but less technical than with the Concero.
#5 - Audio-GD Ref-5.32: Mine was faulty so I couldn't really enjoy it but I could discern a similar level of technicalities than the U3 + Quad. Sound was too dark for me though (w/ the 500s back then)
#6 - Fostex HP-A3
#7 - Audio-GD NFB-3.1
#8 - Emotiva XDA-1