Modifier values modify the base number of a power, so the ATs themselves do not have base numbers. So, for instance, if the modifier for Ranged Defense Buff on an AT is 0.1000 and the power being used by that AT has a Base Defense Buff of 1.5000, then the buff granted by that AT using that power is .1500 or 15.00%.
To see this example in action, check out City of Data. If you look at
Deflection Shield as used by Defenders, you'll see that the power grants 15% defense. If you select "Unspecified" for the Archetype under Power Effects, that number changes to 1.5000. And if you look at the
Ranged_Buff_Def modifier table, you'll see the Defender modifier is 0.1000.
It was only recently that we added the modifier data to the AT articles, so we haven't done much with it. It might be nice to link to an article (perhaps "Archetype Modifiers") explaining how those numbers work. We just haven't gotten that far.