The Reverend wrote... So is there any disadvantage in using Software volume control? ...
Signal to noise ratio.
You can't connect a 0 db output directly to a fixed gain amplifier without some sort of volume control.
If the volume control is part of the device (software or hardware), then reducing the amplitude of the analog output in the presence of a fixed amount of digital noise makes the SNR much worse. The higher the gain at the power amplifier, the more you have to reduce the volume at the source... and the more noticeable the noise becomes.
This is part of the reason why a 20-watt/channel amplifier with low input sensitivity can sound better with a Raspberry Pi and DAC+ than a 100-watt/channel amp with better specs (the comparison is only valid when driving the same speakers, obviously)
If using a separate preamp with volume control is an option, it will probably produce better results.