Beachbum,
If you are firing the laptop's headphone out into the keyboard's auxiliary input, or into one of the amp's inputs, there is not enough power to do much at all. You'll need to beef up the signal. The only economical way of doing this is using a headphone amplifier device, which is available at Radio Shack. The last time I checked they sold for about $20 and they're battery powered with a couple AA cells. 1/8-inch stereo plug at one end and three 1/8-inch stereo jacks on the unit, plus a volume control. The only other option is to go through a mixer.
Hope this helps,
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)