Hi, Pounce. I searched that manual, too, and couldn't find it. I have a couple of suggestions:
1. If that Denon works like every other receiver that I've used/researched, it will only send frequencies to the sub that are below the crossover point (high pass filter frequency). If I'm correct, then you'll want to set the dial on the sub itself to as high of a frequency as you can. Because the Denon is doing the filtering, you don't want the sub to use its internal filter and potentially filter out any frequencies.
2. If you want to figure out where that crossover is set, you can play a set of test tones that play at a single frequency per tone. You'd turn the speakers off and the sub on, and play increasing frequency tones until you can't hear the sub - that should indicate the crossover point. You could then turn off the sub and turn on the speakers, and replay the tones to see if the point where you couldn't hear the sub is also the point where you can now start hearing the speakers. There will probably be some small overlap of frequencies on the sub and speakers because that filter isn't an immediate cut off, but rather a slope. So, the speakers will begin to get quieter as the frequency reaches that point, and the sub will begin to get louder. Filters that I've seen change at about 12dB/octave. You can download those tones off of the internet for free. They are just sine waves. As a note, don't play them incredibly loud for very long, but playing them at 70-75dB for 3-5 seconds shouldn't hurt anything.
