I find that the 2 PS1400s in L-R position nearly eliminates standing waves in my 3500CF HT room. However they do not quite get the job done for LFE. Adding a L8400p in the middle of the room made a huge difference for LFE, and the 8400p has much better response at the bottom end. That said, I absolutely would not give up having the PS1400s docked to the PT800s in front, since this creates a 4-way design that is as near to perfect for music as I have ever heard.
I don't think that adding more PS1400s would be my preferred route, since other subs are better suited for pure LFE duty. However, if one needs "full-range" speakers in the surround positions for some reason, docked PS1400-Pt800s are hard to beat. But if you want to rattle the neighbors windows with LFE, a dedicated sub or subs will be the way to go. 2 L8400ps, combined with 2 PS1400s will be more than enough to shake any room under 4000CF. I still consider the L8400p to be one of the most under-rated subs around.