mesa boogie has two flagship models: the mark series of amps, which are what mesa boogie was made famous with (used by santana, metallica, dream theater, chevelle, lamb of god, etc) and the rectifiers, which became famous in the early 90's (used by many modern bands such as linkin park, godsmack, nickelback, incubus, etc). the other models of amps mesa boogie has made have not been as revolutionary, even though they are still great amps compared to most things manufacturers put out.
the mark amps tone:
-very tight (ie metallica speed metal riffing)
-great cleans
-very smooth lead tone (carlos santana)
-lot of gain
the rectifiers are famous for:
-deep thick chunky growl
-lots and lots of gain
-not as tight (not good for fast riffs)
-average cleans (although recent rectifiers feature outstanding cleans)
-average lead tone (however, the very old models are very smooth)
when you get the tubes involved in a rectifier, the tone becomes thick and chewy and creamy, kind of like vintage amps would sound when you'd crank them really loud. when you put the silicon diodes, the tone becomes a lot tighter and less creamy. this would be the preferred setting for playing some faster riffs on a rectifier. the bold and spongy settings have the same effects.
does that clear things up a little?