Take a look at the "System framework" image for the DX3 Pro on the Topping website http://www.tpdz.net/products_detail/productId=39.htmlI've come to learn that on Topping DACs the variable attenuation output mode, "preamp" as some call it, is a separate mode and may introduce added circuitry compared to the distinctly different mode "Pure DAC Mode" as described below. ASR tested it as a DAC so they used DAC mode of course.
Here's how one forum guy described it: "The Line Out button on the remote sets the last two output modes: pure DAC only (with a line level output presented on the RCA jacks of the DX3 Pro, or Preamplifier mode where the analogue signal passes through the DX3 Pro's volume control so you can adjust the volume using the rotary knob."
When used in DAC mode, the mode ASR would have tested, the signal is fixed at 2V out. Switching to the variable level control, "preamp mode", allows it to be attenuated downwards from that fixed level.
Here the distinction is explained in one of their manuals and the chart clearly shows that volume control is bypassed when in DAC mode, the only mode ASR tested.
Whereas on every AVR and prepro he tests he is passing the signal through, at the very least, an addition high gain capable preamp stage [with greater than just 2V output] after the DAC circuit [it's not just an attenuation control like on a Topping], if not other circuitry too if he's neglected to invoke "Direct" or "Pure Direct" mode, as it would seem to me he often does [see my comments on his Marantz AV8805 prepro measurements here and very recent NAD AVR review I posted earlier, here].
It shows the RCA's are connected to the OP Amp and isolated from the left and right DAC's. The volume control setting enables/disables additional digital signal processing in the DAC IC, which make the signal more "pure" if you will. Volume is only set by the DAC DSP, and the OP Amps follow. The D50s is the same.