When you amplify an opamp to greater levels, the opamp noise also gets amplified. A higher noise floor equals lower SNR. Additionally, the performance of the opamp differs at different voltage outputs. It's this delicate dance of maximizing performance and minimizing penalties for each application that earns audio designers their keep.
Yes, usually the line output of a DAC has high output impedance. I imagine that the main reason is that it doesn't have to be low, so designers won't optimize the circuit for low OI, but rather current stability. Additionally, high output impedance means that the RCA cable's resistive qualities are negligible. Amps high input impedance, so the cable's contribution is basically nothing.
No. Resonessence devices use digital volume control. The digital signal is mathematically processed for lower output through a 32-bit control path that avoids losing too much resolution. A finely designed analog volume control is a better solution, but that kind of analog control would probably cost $$$ for the volume control portion alone. Please watch this video: