Newbie here.
If I plan on setting the crossover at 100Hz is there any point in getting speakers with a min frequency response below 80Hz?

Always set your crossover 80 hz or below. At that setting you should not be able to hear where your sub is located if it is dialed in correctly.

^ From what I've read, that is disputed. Some say 120Hz is where MOST people are unable to locate the sub. So I figured I'd split the difference and settle on 100Hz.
Anyway, even if the crossover is set to 80Hz is there any point in getting speakers with a min frequency response below 80Hz?

Newbie here.
If I plan on setting the crossover at 100Hz is there any point in getting speakers with a min frequency response below 80Hz?

It depends on a variety of factors. But keep in mind that a crossover is not a "brick wall" that will stop everything below 100hz, (in your case), from reaching your speakers. A crossover is an electronic filter that will start to reduce the sound level below the crossover point logarithmically depending on the filter. A first order filter will drop 6db for every octave below you main level. A second order filter will drop 12db per octave, (in other words, it will drop at a steeper slope).

So, if you are hearing music at a sound level of 50db, for example, and you have your system crossed over at 100hz with a second order HPF, you are still going to be sending a pretty strong signal, (about 38db) to your speakers at around 50hz. It will be a lower sound pressure level, but it will definitely be there... not gone. At around 25hz, you are still going to be sending a signal to your speakers of about 26db, though at that frequency, your sub should be doing most of the work for sources that can reach that low. Actually, this makes it easier for us to integrate our sub to our other speakers, specially our mains.

You still want speakers that can go down, at least an octave below your crossover point, comfortably because they are still going to get a pretty good amount of signal well below the crossover point that is crucial to blending your sub to your speakers.

Ye110man,

From my understanding, most people can't detect where the sub locality is under 80 Hz. DTS doesn't send anything over 80 Hz to the Lfe channel and most Dolby Digital material is within that threshold, but some recordings are, and a few DVD-A's have material that goes as high as 120 Hz. in the Lfe Channel.

I would take bsoko2's advice and hear what Wolfsbane says you will be missing. :)

Suppose the speakers and sub are relatively flat up to 80Hz. A very steep rolloff would be preferable, right? Would that be the ideal setup?

Suppose the speakers and sub are relatively flat up to 80Hz.
Not sure what you mean here? Do you mean the speakers are flat *down* to 80 Hz and the sub is flat *up* to 80 Hz?
A very steep rolloff would be preferable, right? Would that be the ideal setup?
Here is an article that explains the difficulty of matching the crossover slopes to different speakers: Bass Management Woes: Trouble on the Slopes (http://www.hometheaterhifi.com/volume_12_2/feature-article-slope-troubles-6-2005.html). Different kinds of speakers roll-off differently. Also, the filters in pre/pro/receivers are not flexible enough to optimize many speaker types.

If you could tell us what speakers and receiver/pre/pro you are considering, we might be able to help you better.

Craig