After having completed the assembly of the anplifier from spark fun, I hooked the speakers up, attached the aux in and finnally powered it up with the old 18v supply.  However I just got a throbbing from the speakers.  I guessed the supply wasnt supplying enough current so I hooked it up to a laptop power supply, 3A more powerfull and it worked.  However there are still a few problems to be solved, furstly there is an annoying echoing in the playback which I think is due to the humming noise which is made when no music is played (and when music is played but it cant be heard).  After doing some research I have come to the conclusion that the problem is again caused by the power supply, this time the fluctuations in the voltage.  Although the board already has a 1000uf smoothing capacitor, a forum user said that they had added a 10000uf capacitor and the problem was solved.  Also if the volume is turned up to high the speakers just start throbbing violently, not so good.  Again down to the power supply I think.  Until I've solved these problems I will probably just mess arpund with the spektrum annalysis using an aux input from my pc.