Try a Marshall power brake. (attenuator) You can crank it then. 100 watters don't shine unless they are 70%+.

If you're daring, put a rheostat on the AC line. This will cough up a little more umph from the amp. Be Careful!!!!!!!!!!! Max down to like 95 volts. If you choke it down too much, the power supply diodes might cook.

Also-Wonder what cab you have? Make sure you are at the right impedence setting. If I remember correctly, the SL100's had 8 and 16 ohm outs.