Easiest way to figure out the low frequency response of the towers is to just use a tone generator app and decrease the frequency until it starts audible dropping off.
80hz is a starting point, however, I’ve found I get much better blending between the speakers and sub at about 60hz in almost all cases. If the box tuning of your speakers is 45hz, the driver is going to be most efficient and have minimum motion at this point. It’s a myth that crossing over at 80hz somehow reduces power requirements from your avr. Most speakers display their lowest impedance (and consequently highest power requirements) from about 100hz to 2khz, and a good deal of content falls in this band. You get two huge spikes in impedance, one at the drivers resonant frequency, and one at the port tuning, very little power is actually required at these points.
I always run test tones and lightly feel/look at the woofer to see which point it reaches minimum excursion, wherever the box is tuned, the woofer should be almost still at that frequency. I then set that frequency as the crossover frequency, the effectiveness of the port should cover at least 10-20hz above and below its resonance, which works just fine with the 24dB/octave high pass in the avr. I’ve always had very good results doing this.
As always, it depends on the room and how the sub interacts. A higher xover might give you flatter bass, and so might a lower xover. The issue I have always run up against with 80hz xovers is that the ceilings of most rooms are 8’, giving you a boost centered at 70hz across the height of the room, since subs are generally placed on the floor, this sometimes has led to boomy mid bass, since speakers are usually elevated above the floor, it helps reduce this issue.
On the flip side, I’ve also seen it cause a big null at 80hz in some rooms, which is alleviated by using a subwoofer at as high as 100hz.
Just play around and listen, if 80hz sounds better than a lower frequency, go for it, if a lower xover sounds better, then do that. It’s really just sub, speaker, and room dependent, and in a way, another tool for dealing with room acoustics problems.
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