well, what ohm load you have on either amp? If a lead is damaged on the speaker it will throw off the load, or voice coil is burning up. I would pull the subs and check the leads as well as ohm them out and see if there all matching. If that not an issue, do you have a voltmeter? Massive voltage drop could be happening which would explain the magic smoke you seen. An amp will run cooler and not blow the fets if it gets what it needs, I have a 2500rms amp on 1 ohm load, never gets warm. but i dont have voltage issue.
What is your main run of power wire? Where you got your grounds?