sounds like it's time to RMC. Reduce to Minimum Configuration.
Not just peripherals. anything you can unplug. internal hard drives. internal cameras, even fans. I was told an interesting story recently from an apple service trainer, he carries this imac fan with him to training sessions. If that fan is attached to the computer, it jams up the internal SMC usb bus (where it communicates its RPMs and temperature) and prevents the computer from POSTing. I thought that was quite the interesting issue. I've never seen that specific issue before, but I've seem similar things. Optical drives are good at preventing ANY device from booting, for example, I've seen that numerous times. Things you wouldn't normally think are related, turn out to be connected in unexpected ways.
