Invest in good quality gear, have it serviced regularaly.
i used to take a few spare valves i had incase transit upset anything.... New amp is d class slow no need.
Spare battery in bass case, spare fuse for amp and kettle lead, di box for just incase, a few tools like allen keys and truss key just incase the climate is extreme, But thats it. Is it dont fit in the case it just dont keave the house
My gear is all good, my leads high quality.....
The only time ive seen things crap out is when they are pushed too hard, know the limitations if your gear!