With my speakers, Vmps RM 40's, I use a sub only to augment the bottom octave so I have it crossed over at 45hz. It only adds a bit of weight to the lowest lows. The sub does not work much most of the time but it is worth it to me and wouldn't want to be without it as I like to have all the weight of the bottom octave.
Every room with walls in about any house will have modes and these are worst in the bass region. To pay someone to tune it may not be that bad of an idea if you don't know how to do it yourself. Don't expect to hear alot of extra lows because you have a sub. For music with truly full range speakers it's only there to augment the low lows. It's a very expensive thing to do adding a good sub.
Every room with walls in about any house will have modes and these are worst in the bass region. To pay someone to tune it may not be that bad of an idea if you don't know how to do it yourself. Don't expect to hear alot of extra lows because you have a sub. For music with truly full range speakers it's only there to augment the low lows. It's a very expensive thing to do adding a good sub.