Welcome to the forum, Chris! Congrats on the new gear.
What to get next really depends on your desires and preferences.
For example, do you put a lot of preference on video and watch a lot of movies on disc? If so, then a blu-ray player would be a nice addition. I wouldn't focus on buying a Yamaha one unless there's something specific about them that you like. You can get a good Panasonic, Samsung, or Sony unit for $100 or less.
A center channel is a great addition if you have viewers off to the side. If you sit in between your two Dalis, then you might not notice a huge difference when you add a center channel.