The 7706 processor is going to have a much better power supply and better layout for the DAC and discrete analog output stages. The result is better/stronger bass and an overall smoother sound. It will still have the Marantz sonic signature which is full sound with a soft / rolled off high frequency. If you like this sound, then 7706 is a good choice. If you want a different sound, then you should look to a different brand.
