Quote Originally Posted by markw
first and foremost, when you bridge an amp, it "sees" the speaker load as one half it's rated impedance.

That means you were essentially driving, at best, a two ohm load.

Now, the NAD may be rated down to two ohms under normal circumstances, but when you try to push near-sub-sonics through it at extremely high levels, strange things can happen. Add to that that the 4 ohm rating is most lilkely a nominal rating, which means it can dip even lower at times, it's highly likely that you brung it to it's knees.
It is unlikely that the NAD is rated for 4 ohms bridged which could be the whole problem here.

An amp like the Behringer EP2500 is rated for 4 ohms bridged, (2400 watts, mono). You'll notice that is also explicitly rated for 2 ohms stereo. Beware: many but not all pro, (PA), amps are rated for 2 ohms stereo / 4 ohms bridged.