I just put a amp, tower speakers, and a sub in my 05 X2. I put the amp in the front passager compartment with the fold up seat on the floor with risers so that it wasn't sitting directly on the floor. It is also conveniently right by the battery. I used a 5 channel amp, 4 to the tower and 1 to the sub. The sub is the new Clarion marine grade sport 10", the tower speakers are from monster tower with Polk audio marine speakers. I wanted to get the marine grade JL audio sub because the existing JL speakers in the boat sound so good, but I could not get one anywhere. BTW, the Clarion sub rocks, and the polk monster tower speakers are F'n lound and clear!
Lastely that compartment that the amp is in stays dry, but just leave the seat open a bit when the cover is on so that there is no condensation.
Hope that helped!