The bottom line with a sub, the frequencies in your room to your ears at your listening point should be at the same volume. One easy way to set your crossover point is to simply grab a MIDI keyboard and output a Sine Wave in the bass range. 

Without the sub present, start with a bass note in a semi-low bass range. Descend notes slowly, listening to each to determine if they are all even in apparent volume. Soon, at the lower limit of your speakers, you should hear a roll-off. Depending on how fast your subwoofer crossover frequency rolls off (let's assume -3db per octave), when you reach a lower note that seems to be about 3db lower than the note before it, find out what frequency that is, and start your subwoofer cross-over point about there. Fine-tune until none of the notes between your speakers and the sub differ in volume down to about 30Hz. 

I have a set of Polk Audio 2.3TLs from 1989, which can audibly go down to 25Hz with no sweat. So, I know in my semi-treated room, if the notes through my system are pretty flat, you can get this too if you choose the cross-over frequency correctly. 

Hopefully the crossover will be lower than 50Hz. If you need to cross-over at a higher frequency, direction starts to become a problem, and you may need  two subwoofers to better represent accurate bass imaging. To survive with only one subwoofer, unless you are mixing in surround and the sub is your .1 reference, you do not want to have a sub sounding monaural. You shouldn't be able to quickly pinpoint where it is in the room if it's deployed correctly.

Best of luck!!

calibrating with your ears... what a novel idea...  sounds best too!!   I never got into pink noise for daylight relaxation.

Be careful using sinewave tones in the bass range. An oscillator is better for that sort of application rather than a keyboard synth.  Bob Katz suggests using pink noise, it is much better. It reduces the possibility of standing waves and nulls and things giving you incorrect readings on level etc..

The Katz article I referred to is logical and has an organised approach to sub woofer alignment.

Thanks for sharing all that information. I did learn a lot.

I'm still digesting Bob Katz's article, trying to figure out how much of it I could actually try and whether it makes sense if I can't do the full monty - I don't have the competent acoustician next door that Bob keeps refering to, I doubt that I could afford him either.

Meanwhile I did the ARC2 calibration thing and the correction it suggests to apply seems to be a moderate one. I might just go ahead and try using that for a while until I get 2 days in a row where nothing is on the agenda (if that ever happens) so that I tackle that subject once again ...