You need more amps at low speed which isn't necessarily the same as big amps at high speed. You need to dig one layer deeper and look at the amp curves for the alternators. Start by figuring out how many amps you need at idle. Turn everything on with the engine at idle and measure the amps being used. You might be able to solve the problem by turning the alternator faster with a pulley change. If you need a new alternator then shop for one that provides the amps you need at the RPM the alternator is turning at idle.