Just a Cambridge for an Express? Hmm, not sure if I would do that...
I would trade the Express for the Squeezebox Touch. It has a very good DAC, a really good review from the critical Stereophile magazine:
and has a lot more to offer then if you stick with the Airport Express.
Doesn't this review recommend using a DAC other than the one built into Squeezebox?
Dunno how the Squeezeboxes perform now that Logitech bean counters have had a go at them, but the original "Classics" designed by Slim Devices apparently were excellent. The review says that it was't quite at the level of high-end DACs - in the context of this thread, I still read that as "a lot better than Express"!
Just today I talked with a friend that raved about his Squeezeboxes. He has (among others) an older box, Duet I guess, which has a two-way remote with a display. At some point, a software update added the ability to listen to music with the remote (it has a headphone jack). Sounds like a developing platform...