My personal choice would be the Yamaha A-S501, not because you need it for the Diamond 12.2's, but because it'll last you a couple of speaker upgrades down the line - stacks of power - and because it's a bargain at that price.
I used to own a Nait 5i. Fine amps, but I wouldn't suggest one for...
Then you'll probably like 'Just Wrong', the first track on Palladino's album with Blake Mills. Fantastic bass line and also some incredible sub bass. It instantly became on of my favourite reference tracks - just incredible production.
Hard to choose one thing as there have been a few big upgrades. Getting Focal Shape 50's as my desk speakers was a huge leap in SQ from my old q acoustic 3020i's. Everyone should get pro monitors as desk speakers!