what will make the greatest differance in sound.

I can only replace one item at this time. My system consists of Martin Logan Prodigy speakers, ML 336, Monster 2.4 speaker cables, Magnum Dynalab Tuner, Monster interconnects, arc LS1 preamp, California Audio Labs icon/mkii cd player. Feed-back from an earlier forum suggested I replace the preamp, cables and cd player. But which one first? I am considering a ML preamp 380s at some point, would like to know if this will improve the currant configuration and sound?