Differences in AlNiCo magnets/pickups question

[ZMAN]

What they are doing with the 57s with AII magnets is trying to duplicate the tone from a "vintage" or older guitar that the magnets have weakened a bit giving you a more mellow tone. Fender has done it with their Classic Vibe series using A3 magnets in there 50s series. A lot of guys really like it because it reminds them of older 60s guitars they have heard.
If you look on the Gibson site they have a graph showing the relative pickup output for all of their pickups. You will see where the 57s come in compared to other Gibson pickups. If you are looking at GFS you should try to match the output of the 57s say around 7 to 8k ohms, neck and bridge respectively.
http://www.gibson.com/en-us/Division...ade%20%281%29/

[Retro Hound]

This is from the book The Guitar Pickups Handbook by Dave Hunter

Alnico magnets
II weaker magnet softer and sweeter
III soft but richer and slightly bolder
IV noticeably punchier more definition in the highs
V bolder, cleaner and more aggressive still

Of course the shapes of the magnets also effects the sound.