Amplifier output wattage and speaker wattage
There are two criteria your speakers have to meet:

Their impedance must fall within the acceptable range in your amplifier's spec'.

Their RMS and peak power must be the same or higher than the amplifier's RMS and peak power output.

Bear in mind that having the volume anywhere below maximum is delivering less amplifier power to your speakers. So you may well be driving them with much less than 6 W all the time anyway.