Unless you have a large room I would stay away from the lower end REL subs for a 2 channel music system. The S5's on down, I believe all use a downward facing passive radiator which helps the sub achieve lower frequencies and deliver a nice rich fat tone but it can also create definition issues in the low mids and can be . REL's also do not have any phase control, just a polarity switch. Phase control is very useful if you have two or more subs. I would also recommend using a very good crossover but most audiophiles have a very strong aversion to them even though a good crossover with time alignment capabilities can do amazing things for your soundstage.
As far as room placement any reputable sub manufacturer should have placement information on their websites including REL. I would also check out Soundoctor.com for highly detailed information about subwoofers and their placement.