My conclusion would be that the bad SNR of a noisy DAC can't be removed by an amplifier, no matter how good. Of course, in the case of a perfect dac with a noisy amplifier, the amplifier would add noise to the perfect signal (that is rather obvious). So the DAC and the amp should in theory have similar noise performance so that "no performance gets wasted".
post #2 sums it up perfectly.
I will get into the theory and say Yes and No(in practice).
For instance practice will show that all recordings out there have noise levels exceeding your theoretical -100dB so the contribution of the DAC will be negligible in reality.
Also the amp may have a volume control which changes situations.
When volume is controlled digitally your theory is correct.
Suppose the DAC has -100dB noise floor (which is good and not crappy) and we have an amp that does not add distortion nor noise.
In practice you will not be listening to 100dB 1kHz yet the DAC outputs that.
In reality you will be using the volume control of your amp you will be attenuating the noise + signal and the noise would thus drop in level when the volume is turned down. You keep the 100dB S/N ratio (with a perfect amp that, again in reality, does not exist).
In case of digital volume control (or the DAC being perfect and amp adding -100dB) the situation alters.
Noise will always be at the same level and only the signal will change amplitude. This will alter the S/N ratio depending on how loud the music plays.