As far as I know, Yamaha
seats are iron. When matching two parts, such as a valve and seat, that run against each other, the important thing is that the two materials create a low level of friction between themselves, and that neither one wears the other appreciably. If the materials are chosen well, they can both be hard, and yet there will be little friction or wear.
Conical springs can be used to address harmonics problems, but are more often used to either create progressive rate springs (which would be used in a Honda
to reduce seat pressure without giving up the max pressure at full lift, reducing the load on the seats), or to get more coils into a spring of a fixed length without coil binding at full compression. The first of these is the reason I've heard given for using conicals on a CRF, but I've also heard the opinions of a few good engine builders that this was the wrong approach to take with that engine.
Harmonics are unlikely to have anything to do with accelerated valve/seat wear. What this term refers to is that any spring will vibrate at a specific frequency, and if the engine should subject the spring to vibrations at a frequency that harmonizes with the natural resonance of the spring, the spring looses its tension almost completely for as long as that vibration is continued. The result of this is usually the loss of control of the valve, called valve float. Dual springs are one fix, as are coils of flat steel wound inside the spring. Yamaha
resolved this issue by paying careful attention to the manufacturing process (once again), and making certain that the natural resonance of the spring is at a frequency that the engine won't be able to match.