Try the speakers/sub with various crossovers, higher or lower, and use the one that sounds the best to you, as was mentioned 80hz is a good starting point and the basis to start from.
Also try the speakers set to small and large as well, when set to large they will play "full range". Also when set to large you may have to enable a setting on the Onkyo that allows the sub to receive signals when the speakers are set to large, double bass or Main + LFE or something similar. Doing this allows the mains to add some bass to the room as well. Some people prefer this setting other's don not, so experiment.
As far as speaker mounting, unless you place the speakers on a shelf you will have to put a screw into them or at the very least use a strap to hold the speaker onto the mount if the mount has a platform to rest the bottom of the speaker on, kind of like an old CRT TV wall mount.
