Manufacturers such as Meridian, NAD and Cambridge are now marketing products termed a “Digital Pre-amp” which, essentially, allow source components with digital outputs, e.g. coaxial SPDIF, optical TOSLink, USB, HDMI, to interface with a power amp. There is no provision for analogue components.
But then, to confuse the categories further, some of those DACs with built in pre-amps also have analogue inputs. Look at Benchmark, Mytek as well as the aforementioned NAD. Even Linn's DSM would come in this category. NAD and Lyngdorf (and a few others) blur the line even further but amplifying directly a digital signal without first converting to analogue.
Essentially a pre-amp require a volume control but why is the implementation a factor?
Implementation is important and its important to understand what limitations each implementation has.
A digital volume control for example, attenuates the signal without attenuating the noise, so if used at extreme levels then any noise produced by the DAC can overwhelm the required signal. On the other hand even the best analogue volume controls have an element of noise and distortion inherent in their design; but when used the analogue volume attenuates noise as well as signal.
And talking about digital pre-amps which have analogue inputs, do those inputs go through an ADC and then are treated as digital sources, or do the digital signals get converted to analogue before all going through analogue pre-amps. While I would always argue you need to judge a product based on what the end result sounds like; there is differences and those can be important.