As speaker cone size increases then the lower the frequency range it can reproduce, but the larger the load it puts on the amp driving it compared to higher-frequency speakers.
The issue will all speakers is that the impedance isn't constant. The amount of power sucked from the amp varies with frequency and something called phase angle. When decent Hi-Fi mags review speakers they produce graphs which trace the impedance and phase angle as it changes with frequency. The result often looks like some kind of mad roller coaster ride as both values vary quite a lot. Thinking about just the impedance for a moment, your 6 Ohm nominal impedance speaker can look like a 12 Ohm load at one frequency, and like a 2 Ohm load at another. The lower the impedance gets to zero Ohms then the more the load resembles a short circuit. That's death to any amp running at volume. This is why we avoid running two or more speakers in parallel.
Phase angle you can think of in terms of efficiency. The closer to 0 degrees the phase angle then the more efficient the power transfer to the speaker. As phase gets closer to 90 degrees then the worse the efficiency.
A good visual example would be pushing your car along a road. The closer you are to pushing in the same direction as the car is pointing then the easier the job. This is a phase angle of 0 degrees. Try moving the car by pushing on the passenger door at 90 degrees to the direction of travel means you'd waste all your energy pushing in the wrong direction.
Okay, so that's all the 'sciency' bits taken care of; so what does this mean for you and your system...
You're absolutely correct that a passive sub is just a speaker. What I've illustrated above though is that speakers aren't simple resistors; they place a more complex load on the amp, and the larger the speaker then the bigger that complex load becomes.
At the moment you haven't said what in-ceiling speakers you're running, but if I make some educated guesses based on what you've written then I'd say they're somewhere between 4 and 8 Ohm each which gives you 8 to 16 Ohms per channel with a pair wired in series per channel of the 5.1 Onkyo receiver. That's okay because the amp will be rated to cope with a 6-16 Ohm load per channel.
Reading between the lines, you intimated that the Onkyo sub makes a useful contribution filling in the bottom end for some of the space you have to fill. By that then, and knowing the specs of Onkyo passive subs, would it be fair to say that the inceilings don't make much bass on their own? This means that they're either quite small drivers, or maybe speakers with a 5"-6" driver but fairly small magnet assembly which limits their bass.
Onkyo passive subs will get down to a useful 45-50Hz before the volume for lower frequencies starts tailing off quickly. For some models they claim 30Hz, but that's not really achievable from a 6.5" cone, so what they're measuring or quoting (more likely) is the port frequency.
Adding an additional 6 Ohm load on the amps sub channel by wiring in series won't kill the amp, but it will limit the maximum volume. What you need to remember is that the power supply in the amp isn't unlimited. It probably draws about 300-350W from the mains at full power. If we allow a 10% margin for a bit of heat loss and running the pre-amp and processing features then there's roughly 270-310W available at the power supply taps. Transistor amps are roughly 60-70% efficient which means we have a theoretical 160-215W in total to split across 6 channels. The result is around 25-35 Watts per channel.
The Onkyo subs aren't so efficient. I've seen figures at 82dB/W/m. Depending on the efficiency of the inceiling speakers then the sub channel is going to draw 2-3 times the amount of power compared to each of the main channels. That's not a problem if all you need is a bit of background music, but it's a bit more of a concern if you want to have the sound loud enough to be heard over the chatter of a party.
You see, the design of all consumer gear is driven by cost. In this case we're interested in the specification of the output transistors on the subwoofer channel.
It was designed for a single 6 Ohm speaker. You're planning on doubling the impedance load which means that the volume will drop by half. Having then to run the volume control higher in order to to create an acceptable sound level in the room, and that means using more Watts. If we start to get close to the power limit of what the transistor pair can pass continually then the first thing you'll find is audio distortion and the amp running quite hot. Push it too far and there'll be a "plink" sound followed by silence and a little smoke signal marking the amp's unconditional surrender
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