Well, I have to agree with Joe and disagree with Joe as well.
I will start with the issue of changing to underdrive pulleys in an effort to save power. The power consumption savings are pretty nil these days. So, the problem is as Joes states that it is in that the pulley is not spinning the alternator fast enough.
Now on the disagreement level, larger capacity alternators do have higher charging rates at idle than lesser ones. In fact about 50% of the rating is generated at 1750rpms of alternator shaft speed with the provision, AND THIS IS THE BIG IF, that you have enough shaft speed to turn the self exciting alternator on!! If not, he is absolutely right on....the charging system is not working because of alt shaft speed.
And..not to mention I bet you have some subwoofer moving amp that requires wattage of a nuclear power plant to move and the two makes for problematic conditions.
YOu need to size your alternator and battery to the load that you have hooked up and make sure that the charging system will still charge under the most dire of circumstances or else you have what you have!
ConceptOne Pulleys and Brackets