I think it depends somewhat on the type of amp you're considering. Based on my similar situation, my DAC made a bigger improvement for me. I had a Marantz receiver driving Sonus Faber Liuto monitors. I bought a separate Emotiva power amp and used the Marantz as the preamp. There was a slight improvement, but not what I was hoping for. I think using a receiver as a preamp can limit the benefits of a separate power amp. However, you're Yamaha is newer and a better model than my Marantz, so it's possible you could get more out of it as a preamp than I did in my system.
I sold the Emotiva, and bought an Arcam rDAC, which made a much bigger difference. And that's an entry level DAC for under $500.
Recently, I decided to get my Marantz completely out of the two-channel chain. I bought a Krell S300i integrated amp, and that's made a huge difference. I only use my receiver for movies now, with the Krell doing it all with music.
I assume you are playing lossless files through AppleTV? If you're still using MP3's, moving to lossless files would be a great way to improve sound quality.
I sold the Emotiva, and bought an Arcam rDAC, which made a much bigger difference. And that's an entry level DAC for under $500.
Recently, I decided to get my Marantz completely out of the two-channel chain. I bought a Krell S300i integrated amp, and that's made a huge difference. I only use my receiver for movies now, with the Krell doing it all with music.
I assume you are playing lossless files through AppleTV? If you're still using MP3's, moving to lossless files would be a great way to improve sound quality.