Maybe the thump is coming from above 60 to 80Hz. Have you tried increasing the gain at higher frequencies? Or maybe the 600's limiter is kicking in.
Nope, I didn't try the 80-100hz area. However, I have had a car sub setup with an amp that had a parametric EQ (JL Audio), and I found that I got the most pleasing results by bumping the 60-80hz area.
Linear it wasn't, but gawd it sounded good!
Good point about the limiter possibly kicking in. I wouldn't know how to tell if it was. I wasn't testing at very loud levels, but I don't know under exactly what conditions the limiter does its job and to what extent.
