2. Yamaha's preamp is supposed to be based on a cheapo chip and not comparable to a dedicated preamp. They spent more money on the power section of the AVR. Even a buffer preamp can do a better job, IMHO.
3. So is the preamp in any entry to mid level DACs; ok for headphones not for power amps.
4. A decent integrated amp is usually better than AVR's amp of 2-3 times the price. There is no experiment actually needed by comparing them.
5. I suggest you go for NAD 356 amp with inbuilt DAC that is comparable to all the external DACs mentioned here.