desI agree that it is more likely that miniDSP will continue to develop products with better analog performance than it is that Topping/SMSL will develop a DAC with DSP. And they certainly have been doing this as everything since the SHD has been a step change compared to their early offerings both in terms of analog performance and usability (i.e display). Topping/SMSL also seem to have such poor technical support that I cannot imagine them producing more complex products and assuming the additional burden of supporting those products.
However I think you are a bit off on analog volume control. One of the few reasons to have DACs with such good SNR / SINAD is that you can attenuate digitally and still retain good SNR at low levels. Apart from a few very low noise preamps (Benchmark LA4, I would say Topping Pre90 but the low balanced input impedance is a joke) it is almost guaranteed that a digital volume control will give you better SNR at attenuated levels. Your XDA-2 for example has a spec'd SNR of 113 dB (presumably at 2 V nominal output). If you run the numbers compared to a modern DAC with 120+ dB SNR at 2 V I think you will be disappointed even at attenuated levels.
For example plot below models system SNR for a DAC with 120 dB SNR at 2 V coupled with a very high performance amplifier (Audiophonics HPA-S400ET in medium gain mode), blue trace uses digital volume control and orange trace adds an analog volume control with 113 dB SNR at 2 V. Digital volume control wins at all volume levels because the residual noise from the analog volume control is so much worse.
Michael
I am not an expert in the realm of electrical engineering, I am an acoustics guy, So very much open to correction here.
I would assume that a digital attenuation (i.e. before the D>A conversion takes place) would allow the inherent idle noise floor of the converter to remain at a constant level regardless of volume position. Thus, as you decrease the output volume digitally, the noise floor remains constant; therefore, the ratio between noise and the signal shrinks, and you lose "performance"
In analog volume control (referring to attenuation post D>A conversion, (obviously many use a "digital" encoder to control a resistor matrix), would not only attenuate the signal but would also (in a "technically ideal" sense) attenuate the noise floor of the device.
Let's say we had a theoretical digital domain volume-controlled DAC with a -120db noise floor at full output. Let's say you turn it down to -40db. Now your noise floor would sit around -80db below the 40db attenuated signal. So you've theoretically lost 40db of SNR due to the fact the main signal has been attenuated but the noise floor has remained constant.
If we had an otherwise identical analog volume DAC that also managed -120db SNR at full output, lowering the output by 40db would not only (theoretically) lower the main signal to -40db but it would also lower the noise floor down to -160db (compared to full output). So in a theoretically perfect world, your SNR would be retained.
The issue I don't quite understand with the graph is how the "analog" volume control is tracked at a parallel slope to digital. You'd think that any introduced noise of including an analog attenuation circuit vs Digital would be at a fixed/constant level.. Assuming the noise introduced by the resistor ladder is above that of the actual DAC, you'd theoretically expect a perfectly straight line for analog, showing that the analog attenuation noise is the "bottleneck" per se.
I would have thought the graph would look something like this. the digital DAC outperforms the analog DAC (at close to max output, since the analog resistor stage might introduce low-level noise over a direct digital DAC) and then once the SNR of the digital volume DAC degrades, the analog volume DAC quickly outtakes it, being limited by the bottleneck of its analog resistor stage.