What happens if you have the high and low level inputs connected simultaneously and they are both being fed at the same time? Struggling to get my head around that as surely it’s getting a signal from the AV receiver pre out and the speaker output of the amp.
I think the best way to understand this is to build up a picture based on a few hypothetical situations, then you'll see where the idea has come from and why.
Let's start with adding a sub to a plain simple stereo amp. The amp has no sub out, so the only connection is via the speaker terminals. The audio output frequency from the amp's speaker terminals is full range - say 20Hz to 20kHz - but the main speakers only go down to 45Hz, so the job of the sub is to fill in the remaining bottom end. So far so simple.
There are various different ways that a sub could connect at speaker level, but in the case of the BKs and MJ Acoustics as examples, the sub connects in parallel with the main speakers. That means there are two wires going to the red positive left and positive right terminals, and two wires going to just one of the black terminals. This way the sub gets the left and right feeds plus a 'ground' return. At this point you might be thinking that two sets of speakers connected in parallel will mess up the impedance. That would be the case if it was two pairs of 8 Ohm speakers, but the sub has a very high impedance so is effectively invisible as far as the amp and stereo speakers are concerned.
Now the techie bit is out of the way, the sub gets blended with the main stereo speakers using the high level (speaker level) crossover. This provides adjustment for volume, crossover and phase. The result is similar to having a large pair of floorstanders capable of playing the full range audio that the amp is providing.
Let's say you're the owner of a high-end AVR; one that's good for music as well as movies. You want the best you can get for music, but you also want to do the movie thing. The AVR has a pure music mode which outputs just the front L & R speaker channels, but with some stand-mounted speakers and a sub connected via the more usual subwoofer out RCA connection then you lose the bottom end because the sub channel output gets switched off in Pure mode. Running the sub with the high level connection as well as the RCA sub out fixes that.
From the receivers point of view, the main speakers respond like large floorstanders because the sub is filling in the bass via its speaker connection. The AVR set-up has the fronts as Large. The rest of the speakers are set to small.
When playing movies, the AVRs subwoofer out socket provides the bass for the centre and surround, plus the Low Frequency Effects audio (LFE). The front L&R get full range audio (20Hz-20KHz). During the set-up process, the sub gets blended with the main speakers via the high-level crossover. This is done by ear usually. Then, for the low-level (RCA) input, the receiver's set-up mic can be used. Whenever the system is running for movies, or even music in anything but Pure mode, the sub gets two signals: One is the the full-range audio for the front L&R channels, which the high-level crossover then filters down. The other is the bass that's managed by the AVR. The sub plays both at the same time.
So far we've just dealt with one amp, whether it's a stereo amp or an AVR. Next, we're going to have a look at what happens when there are two amps in the system. This would be the AVR and a stereo amp fed for surround duties by the AVRs front L&R channel pre-outs.
The Hi-Fi amp is permanently connected to the main speakers. Any music only sources such as a turntable or a DAC connect directly to the stereo amp's inputs. Sources for surround connect directly to the AVR. The Hi-Fi amp acts as a power amp during movie/surround playback.
When just the Hi-Fi amp is on, the sub gets a feed from the Hi-Fi amp's speaker outputs. When both the AVR and the Hi-Fi amp are on then the sub gets two feeds and plays them at the same time.
The final question which might be buzzing round your head is how the volume levels of the AVR and Hi-Fi amp are set. That's done during the AVR set-up. Once the correct level is found for the Hi-Fi amp acting as a power amp, then the volume dial is marked so that it can be returned to each time the surround system is used.
