To my knowledge (perhaps there are small differences I’m not familiar with that Ted can enlighten us on), the volume control in the DSJ and DS Senior are the same. They’re completely lossless digital attenuation.
The real difference is in the output stage. The DS Senior uses a high performance passive audio transformer for the final output stage. And this output stage is just so cool. It galvanically isolates the DAC from the outside world AND it works as a gentle low pass filter for the signal. No brickwall filters like you find in other DACs, just a simple, elegant solution that works wonders.
Due to cost constraints we couldn’t include the same passive audio transformer in the DSJ. The DSJ still uses gentle filters instead of brickwalls, but when you listen to a DS Senior vs a DSJ, one of the biggest differences you’re hearing actually stems from the output stage.
There are other differences for sure, but this is one of the big ones.
… Hopefully I didn’t swing to far afield there in answering your question