+1. I've got a few G-400's, and the C5 or C8 is a great bridge PU for that guitar design. I tried a Custom and didn't like it, and later found out the culprit was the magnet, not the PU. Too harsh, bright, and aggressive. Too "in your face." But with an alnico you get more character & color in the sound. The C5 is the most highly recommended bridge PU for SG's here.
The best advice you'll get here is: "Work with what you have." Don't chase your tail buying one PU after another, hoping that one will eventually work for you. Magnets are cheap and change the EQ & output of PU's; Duncan also uses the Custom coils with an A5 (C5) and an A2 (CC). You can make your Custom a C5 by taking ten minutes and putting an A5 in it. Some of us like the wildcat C8 (Custom with an A8 magnet). Odds are you can get some great tones from your Custom with a magnet swap (10 minutes and $5).
If that doesn't turn out to be what you want, there are PAF type PU's ('59, PG, '57, 490, Virtual PAF, etc) that are vintage low output. Some players find them too thin on an SG (but great in a fat LP).
A third option is medium output PU's, and Dimarzio has a good selection of them. I just put a Fred (10,000 ohms) in a G-400 this week, with 250K pots, and it sounds great. Full sound with lots of crunch & bite. I paired it with an A5 PAF-type in the neck. If the PGN is working for you now, great. Lots of guys here love them. If it's a little soft in the low end or not as lively as you want, you can try an A5 in that too.