Have you considered the AT-1?
I have experience with the Custom 5 myself and I agree it's a slightly more balanced and even bigger sounding version of the JB. Now, in a lot of cases I find the Norton to be DiMarzio's 'improved JB', although a Norton has leaner and tighter lows, but like the JB, harmonics jump from it.
And the AT-1 builds on that; bigger lows, still medium/high output, good harmonics, just more balanced/easy sounding, without becoming dull. I don't agree with the 'Treble 3.5' rating on the Dimarzio website. Treble is less than the Norton, sure, but not 1.5 points.
Either way, if the guitar can use some added body and warmth (like what a Custom 5 gives), the AT-1 will do this too.
And just like the Norton (and to a lesser extent the Custom 5), it splits beautifully as well.