It seems like there is something wrong with the amp. The lead channel should have plenty of thick distortion. Most problems on Mesa amps are usually tube related. Ask how old the tubes are and if there is a possibility of changing them without you paying for it. If there still is a problem, you can always try to lower the price and then take it to a good tech.
Another problem with that amp is that the gain knob is shared
between the two channels, so a great clean sound may not be the best setting for the lead channel. Try a setting at about 7 or 8 on the gain (pulled) knob and set the treble to about the same. If the amp is ok it should be plenty distorted and thick sounding.
Does the amp have the graphic eq?