If that 55 watt per channel rating is a RMS (continuous) rating with THD under @.5% THD, IMO that will be more than enough to power to enable just about any bookshelf speaker to provide levels in the "typical" living room, or especially dining room, that would completely wipe out any normal conversation i.e. you would have to shout to be heard. In my estimation this would happen with speakers in the 86dB and higher efficiency range.*
As far as floor placement: it sounds like you're using these for background music purposes, so I wouldn't worry about good stereo imaging and such, but placing them on the floor will very probably produce a muffled sound quality and irritating boomy bass, partly negating their use as music providers, not to mention the increased chances of being damaged by people's feet or beverage spills.
* I still own a Pioneer receiver purchased in 1983 rated at 45 watts/channel - into 8 ohms with .01%THD from 20Hz-20kHz - and it has no problems whatsoever operating speakers I've owned, e.g. going from 86dB efficiency ratings (Advent "Baby" bookshelfs with single 6.5" woofer) on up to floorstanders with single 12" woofers (91dB)