Not all preamp technologies are the same. Most use active amplifiers which add noise and distortion, some use attenuators, which add noise and sometimes HF rolloff. The Overdrive DAC uses a different technology, which is actually superior to a separate active preamp or attenuator. It varies the D/A reference voltage (the height of the stairsteps) in order to reduce the volume. The S/N ratio actually improves as the volume is lowered, unlike all other volume technologies.
Another consideration is the current drive capability or the output impedance of the analog driver in the DAC. Most amplifiers require a bit of current in order to achieve good dynamics, so if the preamp in the DAC has excessively high output impedance, this can be a problem with some amps. Depends on the amp.
Steve N.
Empirical Audio