If there is extra output buffer, we need to assume that the output buffer has less noise than the dac. So attenuate the signal and the noise the same time using step attenuator will be able to preserve the SNR. Opa1612 is easily 0.3uV noise. It will be better than input noise of most devices.That sounds like option (A) from my prior post, assuming that "suitable attenuation" is unity gain. It's a reasonable assumption since the analog ouputs on a DAC chip are typically around 2 Vrms. In this case, when you digitally attenuate within the DAC to 50 mV, you're passing a small signal (attenuated by -32 dB) through an analog stage at unity gain. If most of the noise & distortion comes from the analog path (not the DAC), then you expect 32 dB worse SNR when you shrink the original signal by 32 dB. Essentially, you shrink the signal while noise level remains constant.
Ideally, when you attenuate the signal you want to attenuate the noise too. Since some of the noise comes from the amount of gain in the analog stage (lower gain = lower noise), why not take the full-scale DAC signal (say 2 Vrms) and pass it to an analog stage with less than unity gain -- that attenuates it by -32 dB (gain of 1/40 or 0.025). Now you have the same 50 mV output, but your analog stage has attenuated a full scale signal, so noise & distortion should be lower.
Would that work?
If you use opamp as attenuation then the least noise is 1 times the noise of the opamp itself which works similarly but a bit higher noise in every volume setting.
The way to preserve the SNR is to track the digital volume. Lets say we have a step for every 6db of attenuation. So at -6db it switches from digital attenuation to step attenuator and set back the digital volume to 0db. 6 steps will give roughly 36db of extra dynamic range.
However when the dac has noise of lets say 0.6uV, it doesn't really matter in the entire system for the most part as the next component in the signal chain will likely have higher input referred noise. Wantting higher SNR is not really necessary. It can be done to just as stated above.