Are "vintage" DAC's worthwhile, or is this a tech that does not age well

The job of a DAC is convert the digital signal to an analog signal without loss of information and adding audible distortion and noise. There are DACs from fairly inexpensive to outrageously expensive that can accomplish this and have been able to for quite a while now. It is possible to hear differences in DACs but it isn't easy unless the DAC has been purposely built to distort the signal or the DAC is simply junk. The reason it isn't easy to tell DACs apart is they generally measure better than your other components. Amplifiers, pre amplifiers almost always will have more distortion than a DAC, even ones that measure well. Speakers are the weakest link they swamp everything else. Add your room acoustics to the speakers and it's very unlikely anyone hears differences in competently engineered DACs. If they tell you they can ask if they listened with their ears only or their eyes and ears in other words did they pick a DAC better than chance without knowing which one they were listening to. 

If you can't distinguish the sound between a $100 dac and a $1,000 dac you better get your ears checked and get the wax taken out, and you're an idiot for saying that, obviously your system isn't good enough to be able to hear the difference. LOL

I own 6 DACS of varying shapes sizes and usages, ranging in price from $100 to $500, and I sheepishly admit that I can't hear any differences between them.

That said; I can hear the difference between a budget DAC/amp and a pure DAC paired with a decent amp.

 My most recent purchase was the Gustard X16 because I wanted a decent DAC with Bluetooth for my YouTube friends. 

dacs definitely have distinct sounds... obviously one needs a good, resolving, proper set up system, decent recordings and some listening experience (and good hearing, of course)

dacs are not just about converting 1’s and 0’s to analog waveforms (even this process has several discrete steps)... they also generally contain an internal line level analog amplifier (and thus, also, a power supply to that amplifier)... these output buffers and/or amplifiers, just like all amplifiers, have output impedance which must work with the input impedance of the receiving device -- this is why they sound different, just like different active preamplifiers/linestages sound different

the sonic differences are certainly more subtle than differences between loudspeakers, or phono cartridges, but are more distinct than among, say, different solid state amplifiers or phono preamps