A DAC chip either provides a current (so
no voltage on the output) or it provides an output voltage.
Cheaper chips (simple low parts count solutions) often provide a voltage out.
Easy to use but not top performance.
Internally the DAC section in the chip is probably current based but there is an internal I/V circuit in there (opamp + resistors) which saves costs and board space.
As soon as one simply connects a resistor to the output of a current-out DAC chip, hoping current is simply converted to voltage then non-linear behavior will be the result.
Such a current output must go into a '0ohm or 0 volts' inputs.
This is easily created using an opamp with the + referenced to ground (or a reference voltage from the DAC chip).
The - input will always want to remain at the same voltage as the + input and does so via the Rfeedback.
This means the input voltage (I in below) will never have a voltage on it but does have a current flowing into the Rf which 'sum' to '0' at the - input.
So current from the DAC chip - the exact opposite current (Vout/Rf) sum game.
The input current thus is converted to an output voltage (current x resistance) and this way current is converted to voltage I/V.
The higher quality DAC chips are current out and most of them even balanced (+ and - out).
As audio equipment is all voltage based the DAC chip has to be connected to an I/V converter.
This usually is an opamp where the + input is either biased by a ref voltage from the chip or referenced to audio ground.
The - input is connected to the current output of the chip.
When there are glitches (short spikes) often the right opamp has to be chosen or countermeasures must be taken to eliminate those. Gating or filtering.
There are several possible solutions for I/V conversion (filtered or not followed by summing or not) with op-amps or discrete parts, even using tube circuits which may even be including (partial) post filtering or not.
It is easy to just follow the directions laid out by the DAC chip manufacturer but requires the usage of specific parts and deviating may result in poorer performance.
A lot of audio manufacturers feel they know better or want to do it differently and believe in 'house sound' or use cheaper (less suited) parts.
This is where a potentially good DAC chip performance can be ruined by choice of not suitable alternatives, post filtering or silly designs based on 'theories' of designers.
It is why some DAC devices that use the exact same DAC chip (and parts before it) can have substantially different technical performance.
Design choices or component choices made on whatever motivation of a DAC device manufacturer determine the final result.
This is what is meant with: The analog output stage is important. It is easier to mess it up than it is to get it right. Sometimes even when following exact recommendations of the DAC chip application manuals.
The audibility of this all is another matter though. This could range from 'psychological' to 'real' issues.