Fender cab sim (PCB by morocoto) question

[elic307]

Hi I wonder if any has built this sim and how it sounds. I am at the last stages of building this one http://www.aronnelson.com/gallery/main.php/v/Morocotopo/LHX2-FENDER-CAB-SIM-SCHEM-N.jpg.html
I heard somewhere in the internet demos of the sound and I've heard distortion. I just don't understand how you could achieve distortion with linear components only... unless you're counting on the low (9V) voltage and clipping. Can anyone comment?

Thanks,
Eli