I have Adam s3a monitors and one sub10 subwoofer. The S3as have pretty good low end response as it is, that's one of the things they're known for in fact, but I do prefer them with the sub. I can't afford a second sub right now, but I have placed it centrally between the two speakers with the cone facing the listening position, if that makes sense.
Anyone familiar with the S10: basically simply like many subs it has obviously a response setting - I have it set at 80hz, so it works below 80hz, and you can either filter out that frequency from the monitors or allow it to pass through of course, and then there is a phase button. I let it pass through, using the sub as just an support of very low end. I don't think there should be phase problems with frequencies that low, especially as they are not more than 1 meter from the sub anyway (it's in the middle just underneath them under an open desk obviously with the cone exposed)... but would this be a problem??? Does anyone know if it's better to filter out the bass from the monitors and use it in this supporting way, or is this too inaccurate... - am I losing much by not having the sub bass in stereo?? (it's why I let the sub bass pass to the monitors).
I also found that this position for the bass seems to give the most even response around the room, although I'm most concerned with the listening position of course.