No sound output when Amplifier is Bridged

[famouslee99gt]

Last week I developed some problems with my stereo system. I have a Kenwood headunit, MA Audio mono amp (800 watt mono), and a Sony Xplod sub (10 or 12 inch, factory sealed box). When the amp is bridged, I have no output. If I hook it up to the left side channel, nothing, right side, nothing...however, if I hook it up to the negative on the left and the right, I will get output, that doesn't sound very good.

Did I blow a channel on the amp? I hooked the sub up to another vehicle and it still works. This amp was a refurbished unit but has given no problem in the past 3 or so years I've had it.

[Steve2000]

If you hook the head unit directly to the speakers do you get an output.
If so, the amp is done.
If not, the head unit is not putting out a signal.