Try using a large cone. Make one out of a large piece of cardboard. Make sure it has very little gap between it and the floor. Cover the outside with a bathmat (fuzzy) to block outside sound. Place your recorder at the very top... Experiment from there...
Then ask yourself: Do you really want to hear your neighbors conversations???
Terry
When I clicked on this topic I wasn't expecting such a specific request for advice, but the answer? Right on, those are words of experience there!
I also agree about listening to neighbors, that's Rear Window level stuff there. Unless you care to elaborate on the reason?
edit: Just saw the reason after the post. Don't worry about hiss, they won't hear it from speakers through the floor. Though it may be fun to anonymously mail, or doorstep, them a CD of whatever disturbance they are causing. That is if there are others who may be disturbed and can be suspects. I wouldn't advise taking disturbance to harassment, but anonymous pranking would be great.