He has the

JBL GT4-12

He has a rather small single 4 ohm 12" sub rated for 250 watts. This calls for a 2 channel amp bridged mono if he is not adding a second IDENTICAL sub or if he is not upgrading later on. Going to a mono amp right now can be done provided the 4 ohm power rating on the amp is in the 250 rms, 500w rms 2 ohm mark. Go to the mono amp if you are going to be adding a second identical sub or if you are going to be replacing the JBL with something different.