My friend bought an MTX TD500.1D, which he used for about 3 weeks before connecting the power wire to the speaker terminals. The fuses did not blow, but rather when we hooked it back up it turned on and even transmitted a signal to his woofers. The issue is that the signal is significantly weaker, and when I say significantly weaker it seems as if it isn't actually amplifying the signal at all. The subwoofers produce sound, but it is at a level so small that in the front seat you can't tell they are on.
Now if this was the only symptom I would blame it on the connection of the power wire to the speaker terminals. However, the head unit is now sending a decreased signal to the door speakers. Also, he has trouble starting the engine when an amplifier is hooked up in his car. I have never experienced anything like this before, so anyone with any information that could be useful, I am all ears.