It’s not the speakers you need to take the load off of. It’s the amplifier(s).

It’s amplifiers that have the real issues with low frequencies.

Adding a subwoofer channel will increase whatever frequencies the sub is playing.

But unless you are crossing over prior to the signal reaching the amps, you aren’t going to get much benefit in clarity. Which comes primarily from eliminating slewing sag in amps required to simultaneously produce low and high frequencies .

I just added a sub to 2-channel and on my Marantz PM7000N you can toggle the subwoofer crossover. (I landed on 60hZ) Oddly adding the sub made my main speakers sound worse, more shrill. Then I tapped the Source Direct button (supposed to remove any tone filters and leave the pure signal) and they sounded normal (great) again and the sub was still doing its thing regardless. I was so relieved.

My takeaway: my speakers (Wharfedale Lintons) needed the full signal to sound their best. It felt like the lowest signals the speaker received colored the mids too on these speakers and overall sounds richer

Taking the load off the mains is very important with budget speakers…less important with higher quality speakers…and then you have preferences…I like to crossover at 50hz if I can.

To properly implement a sub you need to roll off those frequencies (approximately 80hz and below) from your mains. If you do this, you are saving your mains from having to do the large excursions needed for those frequencies and allows them to produce a more accurate midrange. One way to do this is to put a filter consisting of a capacitor between the pre amp and amplifier. The cap value is calculated based on the input impedance of the amplifier. You can do this with separates as well as any integrated or receiver with a pre out/main in.

Pretty much the only reason to cut the low frequencies of full range speakers imo is if you’re having phase issues dialing in the sub(s) with them.

Otherwise, neither you nor your gear will ever notice any difference in efficacy.

i imagine that if your speakers go down to say 28hz, they roll off about ~35-38 just for example.

your sub crossover would be around ~40hz. this is about where the whatever bass was there, begin tailoring off in volume and energy.

even without dsp, you can hear the place if you make a few adjustments.

this is letting your mains shine and giving them some energy in the frequencies not being handled by the mains because they either rolled off or dont go as low.

as far as time alignment, i cant speak.

my subs are outboard of my mains with the baffles aligned as best i could by ear. sounds fine tome.

You are correct. One trick I use is to plug the port(s) on vented speakers to force them to roll off a bit, and that will alleviate some of the burden.

No, the CXA81 does *not* apply any high-pass filtering to the main channels when using its sub-out.

The sub output on the Cambridge lacks bass management so the best course of action is to find the specs for your loudspeakers. Set the crossover point on your subwoofer to the lowest frequency your loudspeakers are capable of producing. That’s your starting point.

Now the more challenging part. Setting you subwoofer’s level, or volume is more difficult if your goal is accuracy. You can get somewhat close using Room EQ Wizard or you can play it by ear or adjusting the level to your personal preference. One hint, if the bass sound tubby, lower the level until the bass sounds natural to you.