You already stated that you lost part of your surround stage - when one of the speakers fell.
Need to get it fixed or buy another pair. Now while it may sound somewhat strange to you,
getting a bigger more powerful sub will help, and make a difference.
Now for me, I would prefer that my side and rear speakers be the same speaker, for a more
Also, if you say that you are losing some surround depth - then turn them up some more.
When you are ready to spend, there are other options - with more range.
There will be some limitations due to your open space - however, not the end of the world.
Also, try placing some Blu-tack under your speakers, when placing them on stands.
Who and Where - is the Way, the Truth and the Life?
Speakers > MB Quart VS05, Boston VS260, Snell K7
Subwoofer > Mordaunt Short Aviano 7
Receiver > Tascam PA-R200, Pioneer VSX-30