I had the same development with my Q18. When I first got it the thing was kinda flat sounding but after it warmed up a few times it sprung to action literally. I dont think I will stray from Fi's subs.
Jay-cee is right. one big factor depends on what amperage your car is sucking down from your alt. I know I used a stock 95 amp alt with a D3100 in the trunk on my vic and I stayed above 12.8 at full tilt with 2000 rms. I have one for sale actually