What frequency do I crossover subs

This will depend a lot on the slopes of your speakers and subs. And yes, I agree that an SPL meter would help here. They are not expensive. Can't speak to the quality of them, but there are even SPL meter apps for iPhone and Android.

Most all of the above, but if you are running sub woofers to augument the bass of 2 way speakers, not to relieve the mid-range drivers of some of the load created by doing the bass, try running the main speakers full range and rolling off the subs as low as possible. A SPL meter can help finding the ideal cross over point as well as positioning them to take advantage of, or minimize room nodes/nulls caused by placement of the speakers, subs and mains, as well as the listening position.

Just a WAG, but if your speakers are -3db at 45hz, and your subs have a 6db per octave cross over slope, I would agree with Timlub and try setting the cross over at 40hz or lower. But consider the location of the sub will have a lot of influence on your decisions, its not just the cross over.

This decision also depends on the listening room, the shape of the FR of your sub(s) vs. your main speakers in the lower frequencies, and whether you're using digital room correction ("DRC", like Audyssey, ARC, or similar).

Without any DRC:

It's very likely that your carefully located, single subwoofer will produce notably smoother bass than your main speakers. This is because you can place the sub close to a wall where reflections of low frequency, long wavelength signals (destructive interference) are minimized. If you are using a pair of subs, their output is almost surely much, much smoother in the bass because, while each sub will still have small FR irregularities, those irregularities are likely at different frequencies and will tend to average out to a very smooth summed response (vs bass from main speakers out on the floor). IME, this difference will usually be pretty dramatic.

In this case, cross the subs just high enough to allow the subs to handle response where the room does its worst damage. In an effectively treated room (bass busters, etc), I've found that this will generally be somewhere around 70 to 80 hz, although it's obviously dependent on the characteristics of your specific room. In an untreated room, the worst issues usually persist up to +/- 125ish hz. I'd suggest that you experiment with frequencies between 70 and 100 hertz and see what sounds best.

If you're using DRC, then it's a different story. IME, the software will effectively smooth bass response wherever the subs are placed and wherever they are crossed. In this case, you have a lot of flexibility and the choice will be more dependent on the specific qualities of your subwoofer(s) vs your main speakers. Try it at 50hz and try it at 80 hz and see which you prefer.

Good Luck

Marty

All really good info. I will add this though, the most inaccurate controls I've encountered are the x-over controls on subs. Often, the setting mark is nowhere near actual. Your ears and eyes (take notes), along with patience are most important.

Im intrigued by Marty's point about bass smoothness. My mains extend to about 20 Hz, but the smaller center and surrounds can't do that. How important are the center and surrounds in setting crossover? I use a Velodyne SMS-1 bass manager to control a pair of HGS-15s.

I set the initial level of all speakers to be equal with my RS SPL meter, but trim the subs 6-10 dB because they seem too loud -- I assume the meter is relatively insensitive to LF. I like the level of the subs to be such that you don't identify them as a source of sound.

I've tried 40, 60, and 80 Hz crossover settings in the Oppo 95 (analog) and Cary Cinema 11a (digital) menus. Maybe I should just standardize on 80 Hz.

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