Castle owners What kind of a difference is the...

There are reviews of each @ Hifi Choice (think that they are both in the current review section).

http://www.hifichoice.co.uk/

I listeneed to the Pembroke a few years ago (liked them), but am not familar with the Harlech.

Once owned the Castle Isis and have listened to earlier versions of the Richmond a few times.

The Hifi Choice review of the Pembroke is accurate, IMO, as are most of their reviews on the gear I am familar with.