Well I have colocated 600s, 500s and 500/600s and the extra headroom meant more effortless sound with no change to the sonic signature. Adding a 400 would be different. I'm a big believer in lots of headroom to keep any noise or artifacts to a minimum. The 600, like the DD-18, excelled in this regard and it could easily be argued that it put in the best overall performance of all the subs given the difficult bass tracks we played.

I can try the 600/400 combo in a couple of days. Adding my brothers 600 will provide more headroom but I gather its the sonic signature thats you are interested in.

John