An interesting phrase: "...they seemed to have lost their raw energy, and entered a more melodic phase..."
Like you, I think I have all the Oyster's CD's (except for compilation of tracks I already have) and I've never actually felt their music was ever anything except melodic. Shouty at times, yes, raw at times, yes - but always melodic. I do agree that Chopper's departure has left a gap. His stage presence and performances were always - er - 'enlivening'.
As yet I haven't bought this CD although I have just ordered 'Meet you there'. I suspect that your comments do have a strong strand of accuracy after listening to just the mp3 samples of 'Diamonds', so I'll defer final judgement until I've heard more from it. I'm sure YouTube will cough up a few longer examples.
I hope you enjoyed the concert. It's been awhile since I've seen them live, so I'll keep an eye open for when they're next due around this area. One thing I always respected about them was that they never charged stupid prices for tickets.