Quote:

Adjust the crossover frequency at the receiver/processor to 1.5X the lowest +/-response of the mains

Or set it a couple of Hertz above the 3dB response point, depends on the speaker response and just how fast that dropoff is to a particular brand/model.

Alternatively you can have another individual turn up and turn down the sub crossover while you listen to material and decide where the bass gets too 'muddy' or overbearing (too much bass overlap with the main speakers) or when it gets too thin or lacking (an obvious bass hole).
Surprisingly the results should come pretty close to the 3dB response point. I have my EP350 set at the 55Hz mark with the M60s based on this adjustment and the bass is nice and smooth. Note the the 55Hz mark also conforms to the 1.5x concept.
No noticeable audible drop outs when testing with a bass frequency sweep so i continue to run my M60s as large, full range.

"Those who preach the myths of audio are ignorant of truth."