This is for replacements for my 18" subs and new boxes that I will also need drivers for.Theoretically a larger voice coil can produce a higher magnetic field to drive the cone. But there are different kinds of winding, sometimes multiple voice coils, and all the other differences in the construction. It's sort of like horsepower .. sort of meaningless unless you know the weight of the vehicle, how the torque is transmitted to the drive system, etc.
Generally larger cones have larger coils and magnets. More important would be SPL (measured consistently between the 2 subs), frequency linearity, and overall build quality.
I've got to get a t-shirt made with "Pistonic" .. got to be a hidden joke in there ..Steve has the best advice, when replacing drivers, look at the Thielle-Small (TS) specs. They will tell you if the driver is a match for the box.
As far as voice coils go, there are MANY parameters to consider than the diameter. In fact, there is an industry magazine called VoiceCoil that specializes in driver design. The parameters to consider include:
* Former type (the material the voice coil is wrapped on such as paper, kapton, aluminum, etc)
* Winding type (underhung, overhung, dual layer, etc)
* Wire cross section (round, rectangular)
* Tinsel type (the wires that go from the coil to the driver frame)
* Gap venting
* Thermal circuit
Cones come in hundreds of materials with different stiffnesses and weights and mechanical construction methods, All cones will have breakup modes where the cone is no longer exhibiting pure pistonic motion. The coil size will determine where, when and how the breakup modes start occurring. A lot of this will also depend on the surround and spider materials too.
Finally, when testing coils, one needs to look at the centering, impedance, rocking motions, rub & buzz and other issues. Like with any other product, some companies are better at quality control than others.