Let me try by explaining a few things for you. If any of this is old news I apologize in advance.
In a traditional amp head you have 2 stages the power amp stage that generates power and the pre-amp stage wich shapes the tone.
In Rack systems these 2 are seperate and can be interchanged. such as the Marshall 9200 power amp and a Marshall JMP1 pre amp. Or another comboination could be a Mesa Triaxis and the 9200 or a JMP1 and a mesa simulclass power amp. It all comes down to the users preferances. With such a system the user has a number of channels available. instead of the 2 or 3 on standard amp heads. So you could have 4 different distortion patches available and a few clean tones available at the same time. They can easily be switched by the control pannel or by using a MIDI footswitch. If the user ever got tired of the tones he could simply upgrade to a different pre amp. All of this coup[led with a multi effects processor and the user has infinate combitions of sounds availabble at the push of a button. By using the MIDI footswitch one could set the pre amp to patch number 54 and the effects processor to patch number107 simultaniously with one button. kind of handy if you gig out alot and want differnt tones. Its alot easier and less limiting than stompboxes.
The 9200 is a power amp so you would need a preamp. I suggest something geared for guitar like the Mesa triaxis, the Marshall JMP! or my fave the ADA MP1 or MP2! These can easily be found on EBAY. checkout adadepot.com for more details on the MP1 and MP2. they even have soundclips created by users.
happy hunting!!!!!!