Accuphase E5000 vs Gryphon Diablo 300

Auditioned both and went for the Gryphon, mainly because it's sound "architecture" is bass-dominant, so given your music tastes I'd go for the Gryphon. Another thing is price, the US Accuphase distributor is greedy so you'll be paying more than you should. Another option you might not hear about much on AGON is T+A from Germany, they make great integrateds, can't remember the top model, but you might find it hard to demo unless you live in a big city.

Accuphase can rock. 

As a Gryphon Diablo 300 owner, I would DEFINITELY go with the Gryphon. I auditioned quite a few amps before landing on this one. It simply doesn't have any weaknesses!

Boulder 866.  It's crazy good with a top US company to back their products. 

At this price level you have a lot of very good options. Gryphon, Accuphase, Vitus, Boulder, Pass, T+A, Manley, Line Magnetic, BAT, just to name a few.
 

Given youre talking ‘end game’, the most important thing (in my opinion) for long term satisfaction is to concentrate on the emotional response you get from listening to a particular amp, as part of your system as a whole. Try to ignore price and measured performance (and reviewer descriptions) as these do not correlate with your own personal enjoyment of a piece of gear. If you really want this to be your last amp purchase, then I think you are more likely to be able to step off the upgrade merry-go-round if you demo widely with different topologies (tubes, SS, Class A, A/B, D, hybrid) and ultimately follow your gut (heart) rather than your ear. By this I mean pay attention to how you feel when you’re listening, rather than the level of detail being excavated from a recording, or size of soundstage being drawn. 

Good luck and enjoy the journey.