Multi-channel home theater receivers have a sub output which sends bass below about 80 or 100hz to the sub and they simultaneously roll off the response below that frequency to the main speakers. The advantage is that some of the bass load is taken off of the main speakers and the receiver amps and may allow them to be a bit cleaner in the range that they do handle. A stereo receiver doesn't have such a provision, so it would send a full-range signal to the mains if they're directly connected to it. Some subs have a high-level connection to power the mains which filters out part of the frequencies below 80-100hz before sending them out. Of the three that I happened to mention only Dr. Hsu's site mentions that the VTF-2 has a filter at about 90hz on its speaker outputs. It may be a bit premature to be considering this point before you've decided if you really want a sub. Also, it isn't that crucial a matter and many users run their speakers full-range even when they have a sub
