Well I managed to repair it!! Thanks for the advice with the pad repair.
Firstly I made a little 90 degree bracket out of an old computer PCI slot blanking plate. Fortunately there was a large ground trace in the PCB right by the IC to drill and bolt this.
So now the weight of the heat-sink is supported nicely and should remove the stress from the pads that caused them to lift in the first place.
I glued the lifted pads back down with Araldite 2 part epoxy. I left them for 48 Hours to be certain it was 100% cured.
As for the other pads which had lifted and snapped off, I glued those back down too, then scraped off the solder mask of the trace right next to the break. When I re-soldered the IC I put a small piece of wire across the break in the trace and made sure the solder flowed fully around the wire and onto the newly exposed copper trace, as well as the original pad and IC pin.
Additionally, when I picked up this Dean DGX15 amp, I was informed it has a loud hum and the guitar sound was very quiet.
Well in fact this was caused by a corroded pin 1 of the KIA6280H. Which is the signal input from the pre-amp. So the joint was barely making contact, but still was slightly. This was causing the hum and the signal level drop.
When I removed the IC, unfortunately pin 1 was so weak and rusted it broke off at the base of the IC!!!!!
However I just about managed to rescue it by melting the plastic with the soldering iron to expose the good metal of the pin, just a teeny weeny bit. I was fortunately able to solder a length of wire to this pin (which you can see, the green wire running around the heat-sink). I then soldered this wire to the other side of the board back to where it is supposed to go.
The amp is now assembled back together and is working fine. I had a good go on it for an hour and a half yesterday!