azazel,

I am no expert on room acoustics, but I think that the rockwool will absorb a lot of energy thus decreasing the possibilities of standing waves. Just understand that this is an effect for very low frequencies and that those low frequencies have a lot of power. So it takes a lot of rockwool to absorb the energy and not let it reflect back into the room where it might form a standing wave. There are plenty of online calculators where you can enter the exact dimensions of your room to find out how critical your room might be (I would use the dimensions from one hard surface to the opposite hard surface not counting the rockwool).
Here's one calculator: http://www.bobgolds.com/Mode/RoomModes.htm

High/mid frequencies are a different beast. The surface of the materials becomes a major factor regarding reflection. Say you have 50cm of fluffy rockwool that you cover with thin cardboard, ultra thin plastic foil or a wallpaper: This would take out a lot of the low frequencies but would still reflect much of the high frequencies, because of the flat and dense (although thin) surface of the cover. You can either use a fluffy cover (like felt or fabric) to ABSORB the sound or you can have an irregularly shaped surface to DIFFUSE the sound. Try to incorporate that into your plans on how to cover the rockwool. Standing waves a no problem here.

Regarding your plans for the HTM1 as a center speaker: I was using the 803S with the HTM2 which was a good combination, so I guess the HTM1 will suit your 802 well. BUT: I upgraded the 803S to the 803D and was simply blown away from the diamond tweeter (the increase in clarity was like putting gloss on a picture) and then also upgraded to the HTM2D. Maybe you want to "downgrade" from 802 to 803, but get the diamond tweeter on all 3 speakers (including the center). If you have a chance to listen to the 803D then give it a try.

All the best,
Thomas

P.S. Still no pictures??? frown