Re: DD/JB with an A2 magnet
I would recommend Alnico 4 if Alnico 2 or 3 is not quite strong enough for you.
Note also that Alnico 3 has slightly less overall output than 2, BUT...it's EQ curve is such that it has a little more bass/low-mid warmth than 2.
Alnico 4 is like a "harder" 3 and with more output.
NOTE ALSO that not all magnets of each variety of Alnico are alike, in other words, some Alnico 2's are stronger than others. It is therefore best to have a bunch of each variety laying around so you can choose stronger or weaker versions of each, according to taste.