amp cutting out

I think you're okay with the components you have.  Even though your subwoofer is rated for 400WRMS, that doesn't mean you have to supply it with 400 Watts.  250 Watts is an appropriate amount of power for that speaker.  It's a bummer that you're not getting the full 500 Watts out of that amp that you would get bridging it into a 4-ohm mono load, but it's the best you're going to be able to do without swapping out some of your gear.

Now for the tricky part.  If you run the amp in stereo mode, and put one channel into each voice coil, you have to make *certain* that both voice coils are receiving exactly the same signal.  If you don't, then one amp channel might be pulling the voice coil one way, and the other channel might try to pull it the other way, causing a loss of power or speaker damage.  Someone else mentioned using a Y-splitter to make sure both inputs on the amp get the same signal, but you also have to match the two gain controls exactly.  Here's how:

Get a test CD that has a bass-frequency test tone, like 40 Hz.  Connect up one channel of the amp to the sub, play the test tone fairly loudly, set your gain for that channel and measure the AC voltage across the speaker terminals.  Write that number down and disconnect the speaker.  Now, connect the other amp channel to the other voice coil, play the same test tone at the same volume, and adjust the gain control on the second channel until the voltage matches what you had before with the other channel.  Now you can reconnect all the wires, and you know that your gains are matched so that when both channels get an identical input, they'll produce an identical output.

Scott Gardner

 but you also have to match the two gain controls exactly.  Here's how:

   Only 1 gain on a 2 channel amp isnt there? Good info though

You're absolutely right - there's only one gain control on a stereo amp.  I was confusing this with another install that had a four-channel amp bridged into two channels, each driving its own voice coil.

Thanks for catching this.

Scott Gardner