I was under the impression the A2000 needed for the -5V for the boot process.
The A2000, as with other Amigas, only needs +5V to boot normally. If you want audio output and RS232 serial, then you need both +12V and -12V as well. Most floppy drives and mechanical hard drives need both +5V and +12V.
The A2000 also needs the 50Hz/60Hz tick signal that normally comes from the power supply. It won't boot without it as this timing reference increments some of the hardware timers. For non-original power supplies that don't provide the 'tick' signal, you can change main board jumper J300 from external to internal which drives the timer from an internally derived signal instead.
