Laptop plays audio, but not with speakers??

Hello,
I have a simple but OK E-system 4115c laptop running vista & realtek hd audio manager, The laptop will play sound, but stops as soon as i plug in speakers/headphones. It only has only one audio output which is the speaker out/headphones/spdif out. I do have quite the speaker setup running power amps and pre amps alongside a very large speaker, but this used to work perfect with the laptop & still works perfect plugged into a simple i-pod so im sure this cant be the problem.
I have tried re-installing windows, re-installing new audio drivers and am now pretty much stumped..
ANY help would be GREATLY appreciated.
thanks