Control volume with DAC or Preamp?

And that we'll never know, as it is the graph shows roll off starting at 100hz already and that's with a 100k!!! loading, Stereophile usually also give a 600ohm loading, strangely omitted this time.

Uh, George, you missed my point entirely.

If you look at the graph George posted, be sure to observe the vertical scale as it is in fractions of a dB. -0.8dB down is not bad for a transformer coupled unit. However it should be noted that output transformers need to be loaded properly for optimal response. Given that PS Audio would not know what the unit was driving, its likely the loading is internal in the unit itself. This would mean the external load of an amplifier is likely irrelevant.

From the same Stereophile review:

In each case, the audioband response has a very slight rolloff in the low bass, reaching –0.8dB at 10Hz, but is otherwise flat up to 20kHz.

I’m not sure that one could accurately describe the OP’s direct from DAC to amp application as “passive”?

@unsound Check my original post in this thread.

Ralph is product (his active preamp) protection mode, omitting the reason behind a "why" the "rare" times a passive "may not" be the least coloured, least distorted, and best dynamically

The reason passive controls tend to sound more colored is they are unable to buffer the control, they are unable to control the otherwise audible artifacts of the interconnect cables. They sound best (more bass, more 'realness' across the audio band) when the control is set to maximum but of course this isn't practical. 

George is omitting the rest of Nelson's comments when he makes that quote. He seems to do this as part of his 'product protection' program, since he sells (actually a very good, but also obviously not of his own design) passive volume control. Here is the rest of that passage, starting at exactly the point that George stopped:

I suppose if I had to floor the accelerator to drive 55 mph, maybe I’d think the life was being sucked out of my driving. Then again, maybe I like 55. Nice and safe, good gas mileage…

Is impedance matching an issue? Passive volume controls do have to make a trade-off between input impedance and output impedance. If the input impedance is high, making the input to the volume control easy for the source to drive, then the output impedance is also high, possibly creating difficulty with the input impedance of the power amplifier. And vice versa: If your amplifier prefers low source impedance, then your signal source might have to look at low impedance in the volume control.

This suggests the possibility of using a high quality buffer in conjunction with a volume control. A buffer is still an active circuit using tubes or transistors, but it has no voltage gain – it only interposes itself to make a low impedance into a high impedance, or vice versa.

If you put a buffer in front of a volume control, the control’s low impedance looks like high impedance. If you put a buffer after a volume control, it makes the output impedance much lower. You can put buffers before and after a volume control if you want.

Quite often the volume control in a preamp is better than one in a DAC. This is usually true if the DAC employs a digital control, since digital controls usually shut off bits to lower the volume, resulting in lower resolution as the volume is reduced. There are a few different techniques that DACs use in this regard, but generally speaking, you set the DAC all the way up and control volume with the preamp. This usually allows you more resolution than you can get by driving the amps directly with the DAC, since the DAC (in order to comply with Redbook specification) makes way too much output for most power amps to be driven past overload.