You might be clipping your signal. Also, surface area is a big deal. If you aren't making good contact between the wire and the ground point then all that soldering won't help. Verify that the amp itself has a good grounding point too.

Sometimes, trying to draw too much through a narrow point(bad connection) can also cause a voltage drop and dimming lights. Positive wires need good connections too. It's a pain in the butt but I've seen guys hit 150 decibels on stock alternators without problems