breakthrough of the budget priced fully transparent DAC - when did it happen?

[Chrispy]

And OP asked for how long. I suspect he expected more than "a long time".

Others had already indicated dates....just proposing a question why someone thought it was a recent development. What were his sources of information all this time?

 

[charleski]

The question of transparency will always be slightly fraught. But it's worth reflecting on just how crappy early DACs (even wildly expensive ones) were by modern standards. A good example is served up by Stereophile from 1989, though JA's measurements in the late '80s were still in their infancy and didn't extend much beyond an assessment of linearity at the 15th bit. Unsurprisingly, all these old R2R DACs were abysmal, though some were worse than others (prices are in 1989 dollars):
Accuphase DC-81L ($8250): +3.7dB
Theta DS Pre ($4000): -1.2dB
Sony DAS-R1: +2dB
CAL Tempest II ($3000): -3.2dB

I suppose it's possible to debate how audible these errors were, but I think 'transparency' had to wait for the delta-sigma revolution to fully bear fruit ten years later.

 

[Blumlein 88]

I think you are right. Truly accurate conversion required sigma delta devices.