McIntosh C2500 digital inputs preference

For owners of this particular preamp, if you can share which input you use and perhaps you tried all and one of them sounded better then the others.
Coax digital or optical digital or USB digital?
In my case I'll connect a Lumin U1 mini streamer transport. I could use either of the above digital inputs, but presently only have a coax digital cable, somewhere in midclass.
I would invest in a higher end cable, any format, if I can hear back a couple proven cases.
The C2500 has the latest firmware, DAC works well.
I believe the coax line will have the widest bandwidth but I’m not an expert on these things. I’m still old school. Someone smarter than me will be along shortly. 
I use an Acrolink  digital cable for the coaxial input.  I bought the cable on a close out, retailed for $900, believe I paid around $460. I have an Esoteric SACD player which I connect via the Acrolink to the C2500 coax input.  The Esoteric is wired internally from the factory with Acrolink wire. 
I believe the USB won’t be active unless you have the windows or Apple drivers for it.  Thought I read it in the owners manual.  Not that crazy about the DAC in the C2500, I actually prefer the dac in the Esoteric better, but some Classical sounds better on the C2500 DAC.  The DAC in the C2500 dates back around 10 years or so. 
I didn’t buy the C2500 for the DAC, it was for the flexibility and the tubes.   Enjoy. 
On mine the USB input is enabled, works fine.
Did not buy either the C2500 for the DAC, but it's not bad at all.
This preamp is the most flexible in it's category and I'm utilizing it actually to almost to its full potential.
Just want to sort out lastly this digital thing.