MacBook Pro or PC soundcard

It is better than what you are suggesting though (an internal soundcard).

I've found that a Mac with Audirvana Plus and a decent USB interface is now very close to a disc spinner.

I just thought of something. There are no cd transports with a USB output. They are all spdif (or aes/ebu). Plenty of "excellent" USB dacs, yet no transport mfg has gone with USB. Wonder why? We all know.

It's just wrong ;)

Actually most high end CDPs don't use SPDIF exclusively. A number use I2S or use their proprietary connections with separate word clock.

SPDIF can sometimes result in bad jitter control. FWIW, USB is really about sending SPDIF data over a USB cable instead of a regular toslink or coax digital but with async USB, there's some control over the clock.

Doggie, they use those interfaces because they are better than spdif. Notice how they absolutely keep away form usb as that would be just silly.

I thank the highend transport world in helping make my point. USB is just sily,

*sigh*

USB is a PC based protocol. A transport is NOT a PC. On the PC, you merely use the USB connection to provide an isolated SPDIF/I2S interface (depending on the implementation) as well. Just that it is further away from the noise of the PC.

FWIW, Empirical Audio also provides a USB-I2S interface on their off ramp. If your DAC supports it, it is a better interface than SPDIF.