The Thunderbucker lineup includes a number of available winds. Think of these as the colors on a pallette chart: All the same general color, but varying in depth of hue. These differences are not huge. The design of the pickup sets its fundamental tone.
63N and B: Similar to what Gibson was originally winding. Pretty bright for a humbucking pickup. The B is wound hotter to sonically match the N.
66N and B: Similar to later Gibsons, they seemed to step up their winds as time went on. A 66N is the same as a 63B for all intents.
The 66S is my hottest wind. I tried winding beyond it with selected magnets, etc... Sound clips showed there was no change in tone. There is apparently nothing more to be gained in the fundamental design.
And here is a surprise: My recommendation for a 2 pickup setup has changed. For the most differentiation in tone between the bridge and neck pickup, I recommend the combination of the 63N and 66S. There is a clip that compares this to the classic 66N 66B on the clips page. The difference is slight, but you might like the differentiation.