1) not at all. The LFM-1's owner's manual seems very well detailed, in plain language, on how to set up one or two subs. Let that, and your ears be your guide.

2) none. One of my three subs' ports fires downwards onto poured concrete without issue. Should be no different having a driver doing so.

3) always a good idea to go from one sub to two - especially when the second one is so relatively inexpensive. ie. a good return on your investment.

Jeff Mackwood