Burr Brown (Texus Instruments) and Cirrus Logic produce great DAC's, However i would personally go for the RX-V2500 great GUI interface
I dont think they would make enough differance in this situation, not to mention the 2500 is a mid range as the 659 is the high-up budget receiver. Also depends what your listening to, if you have 2 channel stereo then the DAC's dont have to be taken into consideration as much. DAC's should not be looked upon as having the main cause of how the receiver will sound, it depends on many differant things, such as board quality, layout, power transformers. Have you considered other brands than yamaha? (the RX-V2500 is a great receiver though, expessially for its price if you can pick up a Factory Repack)