Is it worth the hassle swapping when you already have a sub, and bass is the only area there is any difference as far as I am aware? The mid and treble are the same drivers I think.
When I come to buy I will be looking for 803S ideally but I aren't sure I would be if i already had a pair of 804S! The 803S aren't even a lot bigger, unlike the D and Diamond. But I must say I haven't heard them together, just on their own in different rooms and systems.