I am interested in using the Slow filter option on the ADI-2 DAC for its good impulse response, but the rolled off treble in the audible range and imaging down to pretty close to the audible spectrum when using 44.1 kHz sample rate is not what I want.
I fully agree, but in the end the aliasing turns out to be no big issue in real-world...otherwise MQA wouldn't even exist...
And for the rolled-off treble there is a simple workaround: use the EQ to compensate. Here are two settings for Slow and NOS to linearize the treble response at 44.1 kHz sample rate:
Slow B5 G +4.0 F 16.5k Q 2.1 Shelf
NOS B5 G +3.0 F 14.3k Q 0.6 Shelf
As you can imagine with only 3 and 4 dB gain this is just a slight change, it linearizes NOS perfectly, and Slow good enough.
Now the point is that the Shelf filters have only small impact on the impulse response. Basically Slow stays Slow, and NOS stays NOS. See these two pictures, comparing original (red) with EQ'd (blue) result.
Is this something that can be cured by upsampling my music to say 176.4 kHz and then also using the DAC at this sample rate? The manual only shows frequency response for 44.1 kHz, so I don't know what happens at higher sample rates.
From single to quad speed (up to 192 kHz) the filters stay the same, they just double/quadruple in corner frequency (they move out of the hearing range), and the ringing is halve/quarter in time length.
Above 192 kHz the DAC always uses Slow, this is a fixed function.
But the point is a different one: whenever you upsample the data has to be filtered. This filter is exactly the same as when using the DAC itself in Single Speed, and can also be seen in the upsampled result. So you gain nothing.
I am also a bit unsure about the lowest possible image frequency in this case. I think it would be 154.05 kHz (sample rate 176.4 kHz minus 22.05 kHz as the highest possible frequency in my source files), would the DAC even be able to output that?
At 192 kHz there is no real argument against a steep filter, no matter if SD Sharp or Sharp. There will be no 96 kHz signal in your audio material that would make them ring. And even if then you are not able to hear this short, low level 96 kHz signal.