It's not the rise time of the PSU - there's a dedicated PSU monitor/reset chip in the empeg which monitors the supply rails for +/- 5% and resets the CPU as necessary - but, if the empeg is in standby mode it takes very little current, and I can believe the capacitors in the mains PSU powering it for a while. If you did the same trick with the empeg playing music I doubt you'd be able to replicate the behaviour.

Hugo