I do take this very seriously and want to help you attain the sound that you are desiring to achieve. Having spent several years in recording live sound I can attest that microphones and studio effects can affect vocals in a negative way. With female vocals the ribbons can be quite revealing of sibilance and the crossover point /slope / EQ settings are critical to good performance. This is also at the upper end of the woofer's response where the breakup occurs and it needs to be fully attenuated.
There are some latex damping compounds that could be applied to the cone; however, be careful to avoid anything with a solvent that could affect the cone adhesives. A water-based material is going to be the best solution and the same amount must be applied to every driver to maintain the same cone mass.