I had a similar experience but in reverse. I had my system in a second floor apartment and moved to a house where I have a dedicated listening room in the basement with carpets directly on the slab. In the apartment I had to plug the speaker ports to keep the system from sounding too boomy and in the basement I had to remove the plugs to keep it from sounding too thin. From the mids on up the sonic signature was similar, but from the upper bass on down the difference was staggering. There are obvously lots of variables here (although the rooms are similar in size), but the point is if you've got your system sonically where you like it on the slab you might hear something much different on a floating floor.
I'd recommend some kind of moisture barrier and a relatively thick indoor/outdoor (i.e. mold resistant) carpet to avoid using padding in case moisture ever becomes an issue (which, in a basement is not all that unlikely for any number of reasons). If the carpet gets damp or wet you just dry it out and put it back, but padding will have to be replaced. Depends on your situation. Hope this helps and best of luck.
I'd recommend some kind of moisture barrier and a relatively thick indoor/outdoor (i.e. mold resistant) carpet to avoid using padding in case moisture ever becomes an issue (which, in a basement is not all that unlikely for any number of reasons). If the carpet gets damp or wet you just dry it out and put it back, but padding will have to be replaced. Depends on your situation. Hope this helps and best of luck.