The challenge with these types of DACs is that the output stage may not be beefy enough or have the ability to push enough current to drive the input impedance of amplifiers. That is where a separate preamp can come it. Obviously, this doesn't affect all DACs with volume controls, but there are several amps which can be difficult to drive from a DAC.
Of course, the described problem with Gustard X26 hissing may be something completely different.
Of course, the described problem with Gustard X26 hissing may be something completely different.