DAC Connections to streamer?

What’s best connection between streamer and dac - usb, coax, optical?  
Usb can handle dsd but optical doesn’t seem too.  Optical seems limited to 192.  Bandwidth for usb seems far superior vs other connects.  But if using a streamer, seems I can only use optical or coaxial (bluesound) to link streamer and dac.

Do streamers allow a connection of a usb between streamer and dac?  Bluesound doesn’t seem to.
If you use a streamer can you connect to a dac with usb?  

USB seems to be the best choice since it does not produce jitter directly.  It can still induce jitter indirectly by injecting electrical noise into DAC.  You could try USB cable with data line only (no +5V, GND) or/and install optical isolation.   Other than that choice between optical and coax is system dependent.  Coax should be better, having faster transitions, but can inject electrical noise, create ground loops and is prone to standing waves (transmission line effect).  Toslink has 10x slower edges inducing jitter when system is noisy - noisy level threshold translates to jittery timing.

I use Toslink, but my DAC is very quiet and has strong jitter rejection.  When you have problem you can always add external reclocker.  I such case I would run Toslink from streamer to reclocker and then very short (less than a foot) coax from reclocker to DAC.
It depends entirely on the quality of each output from the streamer & the quality of each input in the DAC. So, you'll have to experiment with your own setup.

Some streamers (e.g., Auralic, Allo) do allow USB connections.

I use an Auralic streamer, connected by USB. I've also tried AES3 (XLR) and didn't notice much difference.
Have a chord tt2 dac and only has optical, usb and bnc. Strange no coax.  Can usb cable connect to streamer from the dac? 

Not sure I understand use of usb if not connected to a pc.
Some streamers have USB out to a DAC. Node2i doesn’t. You can get an adapter for BNC to RCA to use coax if you want.

Majority of the good streamers have USB out. Node is not one of them. How much can you afford?
Like idea to use bnc rca female adapter.  Only 1.27 each from mono.  Assume quality not an issue?  Will go nuts and buy 2.

Coaxial bandwidth so much better vs optical.

An Outgoing usb on streamers would be of great value, most like me are so naive about usb connect value between streamer and dac. 

It could very well depend upon the digital input circuits of the DAC.  The best have a USB input that is self powered (not using the 5V from the cable) and have two femto clocks for the families of 44.1K and 48K digital sreams.  Such a circuit should favor a USB input.

That being said, some devices, like the Oppo I use for audio/video, don't have USB outs so spdif or optical is used.  Much to my surprise, I found that an inexpensive Cable Matters Toslink Cable (a Duster recommendation) from Amazon outperformed an expensive all silver spdif cable.  You never know!

Have a chord tt2 dac and only has optical, usb and bnc. Strange no coax.
BNC is coax with a different connector (75 ohm). Chord has been using them for quite some time, as do other manufacturers.

Coaxial bandwidth so much better vs optical.
Optical can easily handle 24/192 with the right implementation and parts (i.e., Chord) and a proper glass cable. Which sounds better? Well, that’s quite subjective and depends on your taste/preferences and, of course, the implementation.
BTW, I use a Sonore microRendu which is USB-out only. Fabulous sound quality and it’s their least expensive in the line.
The new Sonore microRendu 1.5. Is due out in oct for $399. Ie Ethernet in and usb out.

Not sure how it allows connection between bluesound and a chord tt2 dac.  
You can't connect the two. The microRendu would replace the Node 2i as your streamer. While I realize you currently own the Node 2i you asked about streamers which use USB-out and is the reason I mentioned it. You don't exactly seem pleased with your streamer at this point and if your displeasure continues you can always sell the Node 2i and purchase a different streamer with USB-out, like the microRendu. If the new microRendu is better (or equal) to their discontinued model it's a great price.