[vishnu]

Wondering if you feed power to the power rails from bench power supply, ignoring all the power management poweron/shutdown circuit the computer should start anyway ?.

Don't try to do this; the multiple voltage rails need to be correctly ordered to avoid problems with latchup, which can destroy semiconductors.

That's true; if you've got an IC that requires (for example) +12, +5 and -12 volts, and one or two of those voltages are missing, the IC can self-destruct.

[legodude]

Wondering if you feed power to the power rails from bench power supply, ignoring all the power management poweron/shutdown circuit the computer should start anyway ?.

I've done this a couple times to recreate power supplies for various vintage computers. I usually use a 5v supply with some buck/boost converters, but I have also used two bench supplies. I've looked briefly at replacing the Indigo2 power supply with a modern version and it should be possible. The space constraints make it a little annoying, but if you are willing to donate a drive bay to the cause then it would be easier.