Usually trash sound is made by maxing high and gain, set mids to 0 and fine tune the bass knob based on the sound of your guitar...
Dave mustain sound is a bit different thing... a more rich sound using a bit of mids so...
gain=70-85%
bass=40-50%
mids= about 40-60%
highs=80-100%
I dont know well SpiderIV since i have pod2.0 and a toneport but...
i think that the high gain red will fit the job, add a bit of distortion (maybe everything at 50% and tone 50-60% max).
Add a bit of delay to enrich the tone.
Some song has a bit of chorus too... factory setting is a nice starting point.
When it comes to marty leads add another bit of mids and try to low the gain or off the distortion.
I know i'm a bit general but you have to tweak based on your guitar. Bridge humbacker off curse... i will not try it with a fender strat clone!
