Your friend is right in his suggestion to not sheet rock and insulate it. However, my reasoning is that it would just be pointless since the main purpose would be for sound deadening and nothing deadens sounds more than concrete. Either way it would be a "live" wall. By the way live refers to...
^Somewhat agreed. I built a decent 5.1 system a few years ago out of car speakers. I used to do installs on the site and ended up with several speakers and subs laying around as a result. It was as simple as building an enclosure for each driver and hooking them up to a Pioneer receiver...
Sorry I have no recommendations on a professional in your area and unfortunately it is almost impossible to get a bass response that is equal across all frequencies in all parts of a room unless that is taken into consideration during construction You might get lucky moving subs and changing...
When you say boomy I'm assuming you mean that the sub isn't producing the thump you're looking for rather than an unwanted sound. Usually these problems have to do with room acoustics since the size and furniture in any room has such huge affects on sound. This is especially true for subwoofers...