sounding better is all personal preference when it comes to audio or anything. some people might like a subwoofer with a frequency range of 20hz-100hz, and some might like one with a little higher frequency range.
usually amps are rated at their peak wattage. for example, an amps peak wattage is about 600watts, but its about 300watts RMS (constant power). so that means you can supply 300 watts contiuous to a single subwoofer. now if you had two subwoofer, that would only be 150watts RMS for each subwoofer, and technically it would reproduce the same amount of volume and all depending on what level they are turned up to.
i'm sorry but i dont know what a 10w7, or 12w6v2 is, or maybe i could help.
generally i like a subwoofer in the frequency range of 20hz-100hz because it picks up all the tones i like to hear through low frequency. i honestly would only use like 1-12" subwoofer as well. thats enough bass for a car or boat, unless you plan on entering competitions.