I set up with both high rez digital, and 30 ips analog tape. Stereo X/Y pair fed to microphone preamps. The signal from the preamp was split, one side went to A to D, the other side to the analog tape inputs. I could feed any of the three signals to the headphone amp and 2 pairs of Senn HD800s. The listener could select between the live mic feed, the signal coming off the tape, or the output from the A to D/ D to A converter. Most were shocked they could not hear a difference between the live feed and the signal off the tape, but could hear a difference between the live mic feed and the output from the converter.
I find this statement suspect and can only assume a mistake in the setup. For far too many hours of my life, I have heard what has come off the microphones, what has come off digital and what has come off tape. If you have a good quality digital system, the direct feed and digital feeds are indistinguishable, assuming levels are set the same. Tape on the other hand, especially if you know what to listen for, absolutely will have a distinctive sound, even, since I would assume this is portable, 30ips - 1/2" - 2 track. Even with 30ips, in what I assume is a very quiet environment, you would notice, with headphones, subtle tape hiss not evident on the digital feed. I am giving you the benefit of the doubt that at 30ips you were not using noise reduction, since that has a distinctive sound all on its own. If these results occurred, I would go with, poor digital chain, or digital chain levels not matched.
On the master clock front, studios use a master clock for synchronization, not for ultimate quality. Transmitting that clock around the studio and decoding it will impart more jitter than an internal good quality clock. In a studio where you can have multiple ADC, and digital processors, it is advantageous if not essential to have the sampling synchronized. That would apply even more so if you need to have sample accurate synchronization with video. Good studio ADC will have better specs using their own internal clocks than with the master clock generated in the studio. As studios modernize and move to IP (ethernet) networks, master clock domains get smaller and smaller being limited to only local equipment. IEEE1588 over Ethernet provides enough synchronization accuracy for most usages.
For consumer DACs, an external master clock has no practical purpose. It is relatively easy, and inexpensive to generate an internal clock in a DAC, and that will not be susceptible to noise on an interface to an external clock. That also does not handcuff the consumer DAC to a set of arbitrary external clock frequencies that may not be ideal for that equipment.