Originally Posted by Gov
Plus will send LFE material to your mains in addition to your SW. If you run your speakers "small" and have a capable SW, do not set it to PLUS.
Actually, it's the other way around. "Plus" (aka "double bass") sends the low frequency material intended for any non-LFE channel to any speaker set to Large AND to the subwoofer. The LFE material still goes only to the subwoofer. (LFE is directed to speakers set as Large only when the sub is "No" or "Off")
As noted, Plus can't be applied to speakers set to small, as their low frequency signals are already redirected to the subwoofer and Small speakers are unable to reproduce them by definition.
Many people like this effect, but it does not result in accurate bass. Since the amount of bass directed to the non-LFE channels varies greatly by source (each sound engineer independently decides how to do it for each project), there is no way to predict or calibrate the outcome.
To quote an Audyssey representative:
" …low frequency content is sent to BOTH the speaker and the subwoofer. If they happen to overlap in frequency response then you get double the bass—not good. I have it on good authority that this (“Double Bass”) mode was invented for people who can’t sleep at night because their speakers were detected as Small. Now they can keep their speakers as Large and use this kludge to still send bass to the subwoofer."
I'm more inclined to think it was invented for those who can't sleep because nothing comes out of their subwoofer with some sources (e.g. stereo music) when their mains are set to Large, but either way, it counters the intent of equalizing the system to a flat response.