Looking at the specs on those speakers, Id say the issue is not enough power to the speakers... They handle 1200 watts RMS max, so minimum they should be able to handle is 800 watts nominal... You only supplying them with a 600 watt max RMS... You need to find a better amp or you will blow those speakers... Most ppl think too much power will blow speakers, but this is not always the case, under powering them can in fact be worse because the speaker will distort when u turn it way up trying to get the extra oomph out of it... Always best to mate the RMS 600watt subs with 600 watt amp, or the closest u come too it... And with the gain u can always turn it down if it distorts from too much power...