Thanks Technic. I was thinking perhaps that if I ran one stock sub via the conventional right channel through the premium amp It would be adjustable from the oem analyser.
The left channel could run via the 400/4 to give more low down bass which could be adjusted to a certain degree on the amp.
I was thinking this might fill to mid bass hole to some degree. Would I notice the stereo separation on the bass notes? I'm not sure.
Can I run the oem subs through the 400/4 with a 7 ohm rating? If so, I will give it a try