The Apogee Big Ben is an excellent clock. I currently have a Universal Audio 2192 that I use for A/D and D/A and it also has a very nice clock. I am not sure whether or not something like the Lucid would actually be a step up from the Lynx's internal clock, but it may be. The Lucid can be had for pretty cheap but is a pretty solid clock none the less

I have a Lynx Aurora 16, Big Ben, and the AES card coming in later this week for a high end install in a Live Venue up here. If I get a chance I will try and compare the Big ben clock with the internal and the UA.