Delta's website won't show you a particular cabin or seat if that class is sold out. Try the flexible dates option in your search and see if they pop up for different dates/flights. Comfort+ is very much worth it IMHO..early boarding call, dedicated luggage space are great, and being 6'5", the extra legroom really makes a difference on long flights in my experience. I hope you're able to track some down. Amsterdam is fantastic..enjoy your trip!

"We are tied to the ocean. And when we go back to the sea, whether it is to sail or to watch - we are going back from whence we came." ~ John F. Kennedy