Problem : Head Unit Powers But Nothing To Speakers

[chriscordaro]

Old Setup: 99 Concert Headunit, usual bose a4 setup, 15" audiobahn 400 watt amp and sub.

I switched the sub and amp out with a powerful 500wattx2 urban audio amp with a 10" kicker. it hit twice as hard.

The problem: 2 days later..driving down the road..i hear a pop, the stereo turns off and smoke pours out of the cassette slot in the HU. after the incident, the HU powers, but with a blank display. Bought a new concert HU, the unit powers and works like usual, but still no output to speakers..

what are all the possibilities of what is wrong?

thanks