[Holmz]

What does the drive in terms of impedance (ohms)
What impedence are the speakers?

I assume you want more SPL than 50% of the volume knob provides?

 

[Holmz]

What ckirocz28 said is good place to start. Just to rule it out though, have you thought about re-grounding the amplifier to the same ground spot as the 1500.1? I'm not completely sure it's a speaker issue.

One could probably put a DMM on the passive input to determine if the amps are still providing voltage when the sound disappears...

That seems more reasonable than jumping straight onto hallowed ground memes.