Historically they're graded by no as they were introduced starting with a generic low grade Alnico.As the 50's progressed the other grades II,III,IV and V were introduced. AV was developed by the military and was the last to be introduced into the 'public' domain.
Yes they do all add to the character of a pickup although it must be understood that a magnet doesn't have a sound on it's own, it contributes by the way it powers certain frequencies. Alnico II is the softest and generally has a smooth bass and treble although this is more pronounced the hotter the windings get.Alnico III is very transparent, low output and clean,sounds great for rounded fat jazz applications, Alnico IV is probably the best vintage tone IMHO and along with II and III was used in the earliest PAFs-this is a fact and not myth as we've had them analysed and a collegue of mine has also seen original Gibson purchase orders that clearly state AIV bar stock being purchased.The tone of AIV is balanced and extremely organic, it produces the most authentic vintage tone and sits better in slightly hotter vintage winds than AII which tends to get very mushy if used incorrectly. Finally AV is the hottest producing more highs and lows, great for rock applications or where power and cut are important.
Different companies use different grades for personal reasons, we use all of the applicable Alnico grades to suit the correct design, both to be historically correct but more importantly to have the best sound.
Changing magnets in a humbucker can give dramatic results, you soon find the ones that really don't sit right and others that are head and shoulders better.Obviously you can't swap out single coil magnets as they're integral to the coil form.
I've personally spent alot of time voicing all the BKP range with the correct magnets but I do tweak and swap sometimes on consultation with a customer with a specific requirement.
