I am a little confused by the feature on the 990 that allows you to ajust the crossover. what do you do?

Bass management can be very confusing. You mention the 990 but we're in the 970 forum, so I'll touch on both.

Model 990, 970, and 1070: The first step is to set your speakers as either large or small. This is based not on physical size (although that can be an indicator) but on the frequency range that can be reproduced. Personal preference comes into play some here, but unless your speakers can reach down into the neighborhood of 25Hz (which is very rare) it's probably best to set the speakers to small. One exception to this is if you have no subwoofer, in which case the mains will need to be set to large so that low frequency data from the ".1" LFE track and from the other (small) speakers can be steered to the mains.

Once speaker sizes have been set, we can think about the crossovers themselves. If a speaker is set to large, you can disregard the crossover point as it is ignored by the processor. If a speaker is set to small, look at the speaker manufacturer's literature and find out what the low frequency cutoff point is. This will often be the bottom number of a frequency range that will extend to at least 20kHz on the high side. The bottom number (typically based on allowing for a 3dB drop in level, and as a result sometimes called the "-3dB point"). An example: my side surrounds are Axiom M3ti\'s , and the data page for them lists two ranges. One is 60Hz to 22000Hz +/-3dB, and the other is 50Hz to 22000Hz +3dB/-9dB. In that case, our -3dB point is 60Hz (50Hz is the -9dB point, and a drop of 10dB equates to half the volume). We know that the crossover needs to be higher than our -3dB point number (or higher than 60Hz in the example above). How much higher is the question that we have left. One very conservative rule of thumb is to go one octave higher than the -3dB point, which means doubling the -3dB point and setting the crossover there. In our example, that would mean a crossover of 120Hz. I would consider this approach to define the upper limit for the crossover - don't set it higher than 120Hz, in the case of my side surrounds - but you may find that you like the results more using a somewhat lower crossover. I'd try the crossover point either directly above the -3dB point and the crossover point above that (80Hz and 100Hz, in my example case), playing some different demo material (some movies with good lively surrounds and a nice dose of low frequency) and alternating between the two to see what you like.

Model 970 and 1070 - 7.1 direct input: There's one more wrinkle for owners of the Model 970 and 1070, assuming you have something connected to the 7.1 direct input (such as a DVD-Audio or SACD player, or perhaps an HD-DVD or Blu-ray players in the next few months). That wrinkle is the three-position switch on the back. The options are "bypass" (which applies no bass management, in which case the player is in charge of providing bass management), "HPF/LPF" (which stands for high-pass filter and low-pass filter and applies analog bass management using an 80Hz crossover for all seven speakers, in which case the player's bass management should be defeated by setting all speakers to large), and "DIGITAL" (which means that the 7.1 analog input is converted to digital and passed through the digital bass management that we set up above, in which case the player's bass management once again must be defeated). The analog bass management ("HPF/LPF") is the purest approach, although the digital provides more flexibility. If you are using the 7.1 direct input, I'd suggest trying both "HPF/LPF" and "DIGITAL" to see which sounds better.