Going from B&W D3 to D4

[audio_engr]

In that case - how exactly RELs "release them from heavy lifting"?
They play what they play, just like there are no subs - RELs are just adding the "extra".

BTW, I also don't use hi-pass filter.

440-Pro doesn't need to play as loud with pair of subs adding enough SPL in terms of dB in the listening space. The 440-Pro just chugs along ...   
Or else in my listening space thats more than 1,500 Sq Ft, you'd definitely require a 1000-Pro.