Bel Canto DAC upgrade vs Scott Nixon Tube DAC

I also have the Bel DAC2 and the Nixon USB Chibi. I posted my thoughts here: http://www6.head-fi.org/forums/showthread.php?t=202231

The DAC2 definitely has more air, or "breathy" sound. The comparison is possibly unfair because I use a cheap USB sound card with the DAC2, and the direct USB input with the Chibi. I also tried it using a Monarchy DIP with the sound card, and it had little effect.

I'd honestly prefer a sound that combines the two. Would an NOS tube dac do that for me?

this might not be a direct answer to your question but it might provide you w/ some info:
I recently compared my Scott Nixon Saru DAC+ to a DAC2. I would say that the Saru DAC+ had a very comparable sound to the DAC2 except that the DAC2 had a more fortified lower bass. I was running the Saru DAC+ without a de-jitter device. The DAC2 has an on-board de-jitter I.C./re-sampler. Both DACs were using a computer monitor black power cable. Digital cable was XLO ER-6 & interconnects were Groneberg TS Premium. Had to manually swap the last 2 cables each time I switched DACs.
I've heard the Tube DAC a long time back & if my aural memory serves me correctly then I remember the Tube DAC hi-end to be slightly rolled off(smoother?) vs. the Saru DAC+.
The Saru DAC+ is about 1/4 the DAC2 price but it's sound is very, very, very respectable & it more than held its own.
I have read that non-OS DACs get congested w/ large symphonic music. No experience w/ this sort of thing as I listen to little classical.
if you can audition the Tube DAC, you could try this experiment.
FWIW. YMMV.