I use them under my speakers. I also have the 900 and 909. In my opinion you should be carefull with the use of the 999's. In much cases they bring to much "good" things. They easaly overdo the midrange. For me a combination of 900/909 or 909/999 did the job best. Best deal would be the 909's I guess. They do a lot of good things ( also under other equipment !!) and they are much better then the 900,s and they cost not much more. Also experiment with 3 or 4 footers, use them under other components. I use a lot of Harmonix stuff and they all do their job fine but you have to seek were they are in the best place.
btw: I use the Gallo's 3.1
btw: I use the Gallo's 3.1