The high treble inputs might sound more distorted, but there's no more gain going into them than in the normal inputs. When you plug into the lower inputs of each channel though, there should be a reduction in gain available. The high treble channel has bypass capacitors on the mixer resistors and the cathodes inside the circuit which emphasise the mid and high frequencies, which lead to perceived gain (and coincidentally, part of the classic Marshall sound).
If you crank the volume knob, and don't have any master volume installed, you should be able to get the power tubes to distort. With a preamp master volume, only the preamp tubes would get overdriven at low volumes - the control would vary the amount of signal coming from the preamp tubes and hitting the power amp section.
If you want the sound of power amp distortion, you want to install a PPIMV (post phase-inverter master volume). Most of the power amp distortion comes from the 3rd preamp tube, which is known as the "phase inverter" section. It sounds much much closer to a Marshall run flat out than a preamp master volume does. Many people are happy to run these instead of attenuators, and it only costs a few quid in parts.
