jzzmusician - In my experience they can be set and forget after the initial (week?) of set up. I fairly recently got a pair of SVS SB1000. Not the most expensive subs out there but they have greatly increased my level of musical enjoyment. They seem to bring something a lot more complex than just "deeper bass". While their contribution is significant, on another level it is "subtle" (for lack of a better word)....they are sort of figuratively "invisible". Integration was not the nightmare some have experienced. I suspect setting them up to round out the bottom end of full range floor standers (my use) would be easier than setting them up to work with limited range monitors. I do have a complement of GIK room treatments including floor standing corner soffits on the wall behind the speakers/subs.
I set them up mainly by ear but also with some on-line tone samples and a Radio Shack SPL meter. Maybe not the most sophisticated method but it gave me insights to necessary adjustments. Gain, cross over and phase all continuously variable. I don't find myself tweaking them constantly though I sure as heck can hear a lot more of the differences between recordings and even track to track.
Not trying to contradict any of the good advice Erik is giving you. Just reporting my experience.
I set them up mainly by ear but also with some on-line tone samples and a Radio Shack SPL meter. Maybe not the most sophisticated method but it gave me insights to necessary adjustments. Gain, cross over and phase all continuously variable. I don't find myself tweaking them constantly though I sure as heck can hear a lot more of the differences between recordings and even track to track.
Not trying to contradict any of the good advice Erik is giving you. Just reporting my experience.