I don't have one, but the principle is to add some tube euphony to the sound of your setup. MF had a period of making hybrid amps and other kit using both solid state and tubes. The X10-D was so that others who do not have MF kit could get that MF 'tubey' sound. It works in analogue only, there is no digital. It goes between CDP and integrated amp or CDP and pre-amp using phono cables.
It is worth noting that one of the review magazines, whilst agreeing it made cheaper CDPs sound better, it also amplified the signal slightly. A slight increase in volume can be mistaken for an improvement in sound as it reveals more detail and enhances dynamics. But a slight adjustment of your volume control can do the same thing.