I do use an Oppo in my Home Theatre for the last few years, one version back, and have had a few various Oppo's (one the higher 'audiophile' version) brought to my system by their owners to compare to my Playbacks Design. it's not bad, not great.....at it's price it's a good value universal player. in that general price range I'd likely choose the Mytek Digital as hard to beat at bang for the buck at $1695 if you could trade the Oppo at retail. but really at what you can get for the Oppo used it's not a viable way to go to pursue in the context of overall system performance.
I don't know that Cambridge integrated, but again the alternatives after selling the Cambridge used are not likely much better.
speakers is where the needle can be moved the most predictably, and better speakers allow for the 'next' step to make a bigger difference too.
That may be, but with the Cambridge being only 45wpc you'll be limiting your options. What if he likes the sound of a particular speaker that has a very low efficiency rating? His headroom would be greatly reduced, so good luck cranking them up!
Sorry to disagree, but I'm sticking with changing the amp first.