This has been discussed many times, in many threads. Let's try to summarize. When describing the net effect of "Plus" and "large/small", you should also take into account your source. Here is a table I just made that summarizes my understanding:
When setting SW to "No", I believe the .1 LFE is lost, but I'm not sure. It might be redistributed to large speakers, but I never did an actual test.
Here is what the SC-27 manual has to say about the SW setting and its plus setting (p. 125):
SW – LFE signals and bass frequencies of channels set to SMALL are output from the subwoofer when YES is selected. Choose the PLUS setting if you want the subwoofer to output bass sound continuously or you want deeper bass (the bass frequencies that would normally come out the front and center speakers are also routed to the subwoofer).(See note 3) If you did not connect a subwoofer choose NO (the bass frequencies are output from other speakers)
Note 3: If you have a subwoofer and like lots of bass, it may seem logical to select LARGE for your front speakers and PLUS for the subwoofer. This may not, however, yield the best bass results. Depending on the speaker placement of your room you may actually experience a decrease in the amount of bass due low frequency cancellations. In this case, try changing the position or direction of speakers. If you can’t get good results, listen to the bass response with it set to PLUS and YES or the front speakers set to LARGE and SMALL alternatively and let your ears judge which sounds best. If you’re having problems, the easiest option is to route all the bass sounds to the subwoofer by selecting SMALL for the front speakers.
Note: It has been my experience through tests with the LFE sweep of the Avia disk that, contrary to what might be expected, the crossover frequency also affects the .1 LFE channel; freqs above crossover in .1 LFE channel are *lost*. If you disagree with this, test it first with a source that only contains audio in .1 LFE channel!
I believe the above are FACTS based on the manual description, my experimentation with SC-07 and SC-27, and multiple years of following AVS threads. If you believe something to be in error, please say so, but provide references and the AVR model to which your comment applies.
Hope this helps a little to clear things up.
Dan.Edited by info_dan - 3/22/13 at 3:03pm