Bushmaster & Marantz NR1403 AV Amp

I have been a happy owner of a Bushmaster since September. I recently upgraded my AV amp to a Marantz NR1403. Unlike many AV amps these days it has pre-outs so I can drive my B&W main speakers via the unity gain AUX input on my Linn Kairn pre-amp. That way the NR1403 controls the volume. What's this got to do with the Bushmaster?

Just as an experiment I thought I would see what the DAC in the NR1403 is like (the Analog Devices ADSP21487 DAC chip), so I connected my SB Touch directly to it using the optical input. The SBT was connected to the BM via coax and then to the Kairn. This allowed A-B comparisons simply switching between CD and AUX inputs on the Kairn.

The surprising bit is that when using the "Pure Direct" mode of the NR1403 - which shuts off unused electronics - I could barely tell the difference between the two, even when I switched the coax and optical connections. When pushed the BM gives a slightly more 3D sound, but that's it. Without "Pure Direct" mode the sound is clearly harsher.

Any thoughts on why the DAC in this £250 AV-Amp sounds almost as good as the BM in my system?