I did use the SR's for a number of years with success, its only as my system improved and other treatments tried did i become less enamored of them. As for numbers of them, a single would generally be used centered between speakers, best positioning, up near ceiling behind listening position. When I used two, they went up high in rear corners behind listening position. Three, two remained in corners, third in center positioned as previously mentioned. Comparing the three, a single centered can be all one needs, using two in back corners the least favored, left a hole in center image, pulling side sound stage forward, middle recessed. using three brings entire SS forward, can be too much in face presentation. You can adjust how much ss pulled forward by varying intensity/volume knob. I also used a single somewhat satisfactorily placed on front wall near ceiling between the loudspeakers, this raised the SS although over time found this to be artificial sounding. This my problem with the SR's in general, don't present SS in a 'natural' manner, forces SS in the direction of them, even using lowest intensity setting always congealed SS to some degree. This could be phasing issue, perhaps different frequencies respond to SR to varying degrees.
Theoretically these are indeed subsonic 'noise' reduction devices. I never detected any added resolution/transparency with them, likely NR at such low freq doesn't affect freq we can hear.