USB DAC Dumb

david - to me there are 2 questions that need to be answered to address your original inquiry about using an older CDP w/the DAC in your HP amp...1) Can your CDP provide a digital signal that the DAC in your headphone amp can process? 2) If the CDP does have a "Digital out", can you find a way to get that signal into the USB port on your HP amp?  If the answer to both of those questions is "Yes", then you should be able to listen to that CDP though your headphones using the DAC in the HP amp.    If either is "No", you might still be able to listen to the CDP via the HP amp using conventional RCA-type interconnects but I suspect this won't take advantage of the DAC in the HP amp.   Hope that makes sense.

Thanks for the reply.
I haven't purchased the CD player yet.
Of course I will buy one with a digital output.
To keep costs down I'm planning on buying a older carousel player.
The DAC in my headphone amp is modern and reportedly very good.
So, how to use the USB to the digital out in the CD player?

Unless there is such a thing as a coax or optical to USB adapter, you won't be able to.  Have you looked for that?

Mahler's point is Question 2 in my previous post.

D99, Your CD player probably has a SPDIF output.

Sending SPDIF Input to the DAC's USB input cannot be accomplished by a simple cable type of adapter/converter

You will need an electronic converter. 

USB - SPDIF converters are common, but I do not know of any SPDIF TO USB converters

Are you sure your amp does not have any other type of digital input? Such as optical

What amp do you have