i don't know about an amp - i haven't seen one but then again i haven't looked under the seat! in the V5 we had which had the tip top stereo the amp bit was part of the unit under the stereo screen - but there is none of that in the GTI.
its well worth the money, it sounds awesome, and its discreet enough that the speakers won't attract attention to any scum element having a look in your cabin!