I have a subwoofer, an REL 12" self-powered, that I got along with some PSB powered speakers for the TV when I want to live stream concerts. But most of the time the speakers alone are just fine and the subwoofer is just there.
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My current 2 channel stereo has a pair of Allison Three speakers. 10" woofers. They are designed to be in the room corners. I could plug the REL into the NAD C 658 sub-outs and see what happens. The NAD will cut off the Allison's at 80Hz and send 80hz and below to the subwoofer. But I don't think that I can just activate and deactivate them with a switch. I think that NAD switches the subs on if something is plugged into the sub-outs. So comparing with and without means unplugging them and replugging them into the RCAs
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Also, because of the room arrangement (which is full of clutter and isn't going to change much soon) the sub would need to be significantly off to the right by the TV, not by the speakers.
Allison Three Speakers
The two speakers are in the only two corners that work. I can't do much about the clutter at this time. It's mostly my wife's. It is the basement after all, formerly a 2 car garage before it was finished as a room.
The TV is on the other wall to the right of the right speaker. It's the only sensible place for the TV given that the pillars would obstruct view even if I could get the clutter out. The Sub is below the TV on the right side. I might be able to move it closer to the speaker. The Turntable is under the bandana so that spot is out. I'm not very keen on putting the sub on top of the vinyls where the old preamp and EQ are sitting for the moment. I think that at most I could swap locations of the table that the TV is on and the sub but then I wouldn't have easy access to the controls on the back of the sub. I can't rearrange the speakers and put one to the right of the TV. There is a door outside to the deck and a bathroom on that side and then the speakers would be even farther apart. (If anyone is curious, the Amp is a Carver M400 and is on top of the component cabinet with the Star Trek models)
So I see two considerations. Would the REL really make much difference to the sound given that 1, the Allison Three's have good bass and 2, the sub would be well off to the side on the other wall, would it be just too strange? I know that bass is non-directional but still....
At some point I expect I'll just give it a try but I'm curious what people think. It's easy enough to run two RCAs to the sub from the NAD but comparing with and without is not so easy.