Hifonics Amp problem, what could cause this?

A friend of mine who is new to car audio unplugged his hifonics amp and didnt know to remove the connection to the battery when plugging it back up.

He hooked the ground up first then he hooked up the power, it started arc welding inside and he removed it.

I got over there and removed the fuse, hooked the wires up then when i put the fuse in, the fuse blew immediately.

So i assumed he blew the power supply since the stereo wasn't on.

He uses one of those devices to convert speaker outputs from stock headunit into rcas.

So i put my amp in there and everything was fine.

When i took my amp out, we decided to hook the amp up again.. if the fuse blew again... then we knew is was done for...

Well, i hooked up pwoer\ground and remote only, placed fuse in and it didnt blow!

So i then took the fuse out, hooked up speaker wire only and placed fuse back in, still didnt blow!

I was currently jumping the remote wire over to power so the amp was on at this point.

So while car and stereo still off but amp still on, i told him to plug up rcas then turn on stereo.

As soon as he plugs rcas in, the amp generated a constant test tone... He pulled the rcas immediately.

I took a multimeter, one probe on amp's ground, other probe inside rca. Testing resistance, read nothing.

I then touched the outer shield of the rca... As soon as i did this, the amp SURGED and send a LOUD BOOM through the speaker...

I don't know what to think from that so was wondering what could cause this?