For U2 you'd need a ridiculous amount of effects or a good processor. Amp wise - pretty sure the Vox AC30 w/ Alnico Blue(s) is the Edge's weapon of choice. Zeppelin - I'd imagine Page used Marshall.
The great amps under $3000 club: Carr, Swart, Dr. Z, Divided by 13, 3 Monkeys, Tone King...