To hopefully clear ip the confusion here:
The Ensemble iii system is designed for use with a STEREO amp. It's one of Henry Kloss's takes on the concept of the original Bose Acoustimass: greatly reducing the size of the left and right speakers by not asking them to output any bass, shuffling that instead to a dual channel passive woofer that can be placed on the floor. Which works because bass is non directional. This was meant to appeal especially to apartment dwellers with limited shelf/wall space who prefer not to devote the amount of that a typical pair of bookshelf speakers would occupy.
IOW this design precedes the spread of surround sound for home theater. So the premise here is the amp has only two full-range channels, and the speakers are essentially three-ways, just with the woofers housed together in a single cabinet, and the mids/tweets in the separate small cabinets. Strictly speaking, the bass unit isn't a "subwoofer", but Bose labelled it as such, I guess because that sounded 'sexier' or something. But that basic concept of segregating the bass and combining it in a single box obviously became the mode of surround sound systems so the term "subwoofer" stuck. The lingo in the Ensemble manual is confusing now, because by "satellite" it just means the main left/right speakers, not surround speakers.
Mixing components of a system like the Acoustimass or Ensemble with other speakers presents an issue in that the bass unit and main L/R speakers are matched to create a smooth crossover between them. E.G. if you use that Ensemble bass box with different L/R speakers you might get either a hole or a bump in the frequency response curve... which you could address with a graphic or parametric equalizer but might be a problem otherwise.
A surround sound system is a completely different beast because it has a very different kind of amplifier, either 6 (5.1) or 8 (7.1) separate channels of output, with electronics in the production chain performing the crossover separation between bass and mids/tweets.
How you would use a two-channel passive sub like the one from the Ensemble iii with a surround sound receiver depends on how the amp handles bass signals.
A typical home-theater-in-box system e.g. from Panasonic comes with a passive sub fed by a separate power-amp channel from the receiver. In this case you would wire that output to BOTH sets of terminals on the sub. You're probably stuck with the crossover point default in the unit, as the receiver won't have a flexible EQ function. Depending on your L/R speakers it could sound still sound OK.
A typical individual HiFi home theater receiver designed to work with a variety of speakers typically presumed a powered subwoofer. That is, there's no built-in power amp output for a sub, just a line level out. The question then is whether the receiver has a setting to be used w/o a sub. (E.g. my old Sony STR DB840 does). If so, turning the sub OFF sends the lows from the surround sub channel to the L/R outputs. In this case, you wire the Ensemble bass box as shown in the manual: in parallel to the L/R speakers. Then you'd set those channels to LARGE for full range speakers, and adjust EQ to taste.
If your amp doesn't have a 'no sub' option, the lowest frequencies will only be going to that 'subwoofer' line level output, and the only way to use the passive bass box would be to turn it into an effective powered unit by driving it with a separate amp (integrated or just power amp) with the sub output Y-ed to both inputs fir a stereo amp, and the pair of amp outputs wired to the two terminal sets on the sub.
I'll guess such a setup only makes sense for folks on a budget who get the passive bass box super cheap, and already have a compatible receiver or an extra amp in the closet. FWIW, I landed here looking for more info on the Ensemble iii bass box I found at the Salvation Army, and intend to use with a Panasonic head unit from an HT-in-a-box system. I don't have all the parts yet but when I get it together I'll come back and report how it works.
