There certainly is mixing involved thats what Tom does. I think Biffs bang on with his sum up but I'll just say a couple of things to the people here who all think its all coming off a tape or CD backstage.
Firstly - A live sound like the Chems have needs to be split up into channels otherwise it would sound just like a CD playing through a PA. Note the difference between a DJ playing a Chems track in a club and what the Chems sound like live. There's much more dimension to their sound live. Like Biff says a fair amount of what they do during any given track will come off samplers a nd sequencers filtered through Toms mixing desk which he has complete control over. Other parts like those fantastic analogue noises they seem to be using more and more of are done live . Hence the more than subtle differences you can here from gig to gig. For example they added some fantastic little analogue noises over Star Guitar at RockNess when I saw them during the summer, I've never heard them doing that again since with that track. A tape recording I think not.
Secondly. It costs them personally a huge amount of money to ship all that gear around the world when they tour. Do you really think they would do it if it weren't all really needed? Also at festivals over the years where I've seen their roadies set up before their slot I've more than once seen them having difficulties getting stuff patched in. T in the Park 2004 they were late coming on after much faffing with a bit of kit.
I don't think all of that was a put on just to make them 'seem' like they're doing it for real.