There could be differences, more having to do with the difference between a 3-conductor cord setup and a 2-conductor cord than necessarily the construction of the power cord itself. The two conductor cord has a "hot" and a "neutral" conductor. These two connect (through the power switch and a line fuse) to the primary coil of the power transformer that runs everything in the tuner. The question is whether the neutral side is also connected (often through a capacitor) to the metal chassis as well. (The neutral wire in your house is connected to an earth ground at the distribution panel of your house wiring.)A 3-conductor cord adds a separate "safety ground" conductor that connects back directly to the earth ground at your junction box. In modern equipment, this conductor is connected to the metal chassis and the neutral usually is not connected.
Since the Accuphase was designed without this feature, adding it might not improve the sound quality (e.g., it might create ground loops with the dual connection to the chassis). Or, if the IEC connector ground pin was not connected to anything internally to the tuner, then it likely would have no effect at all.
