Loud Snapping when turning amp ON and OFF

I hooked up a new (used) Eagle 2 amp into my system last night, turned off all the sources and turned down the volume on the preamp. When I switched on the amp there was a loud (evil and dangerous sounding) SNAP! then metallic sounding ZITTTT! noise through both speakers. I shut off the amp and heard the same noise, more evident this time through the right channel, followed by a noise sort of like ssssffffttt!

So I checked all the grounds, and changed out the preamp to a reference Line passive. Same noise happened again, so I left it one and played some tunes through the system.

Amp sounds killer, very authoritative bass, but the ON/OFF noise is downright scary as if the next time one of the Celestion SL 600 speakers could turn to toast.

Thoughts? Speculations? Thanks in advance. Kolledog