I doubt that your speakers (not sub) have a crossover to cutt off the lowend. Therefor they require a crossover to provide a match with your subwoofer, and to free your woofers from the extreme stress usually associated with very low end sound (which requires small speakers to move a great deal in distance).
As for an "industrial norm" their is no such thing. This is good because our subs, listening environment, and our speakers all must match up... however this is not a single situation fits all.
I am sorry to hear that you DONT have a any range selection between 50hz-100hz. This is usually where we put it. However, if you probably have a crossover on your AV receiver for the Subwoofer. So you can turn the crossover on your Sub, all the way up, then select it through your AV receiver, through tests and or THX disc, etc... then turn the crossover on the Sub as close to (but above) the setting on the AV receiver.
You need a receiver that has the ability to adjust these values. From the understanding I am getting out of this, is that you have a low cost A/V receiver.
You just purchased a higher end speaker, that wants power... yet your receiver is cutting out on you... Thats what I suspect is happening, though its very odd that you are loosing your tweaters and not your low end.
I would take the speakers back, and have them test it out on their higher end receivers. I suspect you have a problem with the speakers.