Mark,
We have been using the Black lion unit for a few years very good for the outlay for those clients who wish to chase a decent upgarde.
Black Lion Master clock
For those looking towards to one of the very best then the neat studio clocks from the UK guys Drawmer are a very worth while choice.
We have compared them abx with the Apogee big ben, the UK versions is some what more coherent sounding and has improved bass and stage depth too. The Apogee gives a more mechanical feel to the sound. (IMHO)
High quality master clock
Our reference systems use Rubidum ( 2nd level atomic clocks) to slave, the Sever<>Puremusic tarnsporter<>Dac or S/D transporter<>dac.
Do not under estimate the the importantance in an all digital system of 'slaving' all the clocks from a central master, the more pieces of equipment in the chain the more critical it is they all talk at the same time.
The differences are quite something when implimented in a full sorted system.