The plastic sheet over the rockwool doesn't sound right. I'm trying to think of any reason for a plastic sheet over the rockwool... and none comes to mind. I would think the plastic would reflect sound back, diminishing the effectiveness of the rockwool.
I think I have to go back to your original post here and say that you need to be a bit more specific about your environment and more specific about what it is that you are trying to achieve. Simply throwing a handful of diffusers (whether they are great or horrible) won't matter much if your room has other flaws and the level at which you are recording is too loud. Don't forget you have a ceiling and a floor too... those two parallel surfaces are usually a big culprit in creating comb filtering (that annoying

you hear when you clap your hands hard as you walk around the room).
In the end, if you really don't have the time or money to invest in treating the whole room, go with Tiny's reply... treat one corner of the room and put your guitar amp facing that corner and face your guitar amp into the corner.
You can also do a search here and find lots of other threads on this. Or you can go to
www.johnlsayers.com and do a search there. Although I would not put up a post there until you read all their rules (part of the reason I only look and never post over there)
Good luck!