[JahJahBinks]

I have a D100 DAC that accepts AES/EBU, optical, coax digital or USB inputs. I am currently using USB on my laptop and I am quite satisfied with the sound. But I am wondering if it is worth getting an Expresscard to any of the other three input format for better result, is it possible to obtain better result from one of the other three?

 

[obobskivich]

If you have some source material that exceeds the USB ability of this part (I don't know what that ability is), then yes it may be worthwile. Otherwise, no. What *should* be happening inside your device is roughly this:
 
USB -> IC that creates a SPDIF or I2S signal -> DAC -> Output
 
All you'd be doing is potentially side-stepping that IC; that won't do a whole lot unless that IC limits you to something like 16/44.1 (a lot of older USB devices do this) and you needed to playback 24/48, for example. 
 
If you want some processing/enhancement feature that isn't available from software or your existing driver package, then you may also benefit from an upstream device (I could only see this as relevant in a studio or development setting).