IMHO, the main reason so many people have issues integrating subs into their system is that they try to cross them over WAY too high. If your -3dB point of your speakers is 60 Hz, I'd begin experimenting with a sub crossover point of 0.7 x 60 Hz = 42 Hz. Unless you are using a Vandersteen sub, you want the sub to just come up underneath your monitors...and usually not all the way up to the -3dB point of your speakers.nrenter has it right if you are looking to add to what the main speakers can't physically reproduce. That's a good starting point and may need to increased. By setting the sub to 80hz you will add the lows your speakers can't do but also add to what they will do (huge hump in the 50 - 90 hz region). But if the sub is out of phase with the mains you'll end up with a suckout in that 50 - 90hz range. Phasing is very important because you don't want the added lows leading or lagging the lows from the main speakers. I have a Rel B3 which has a phasing switch - 0 or 180 degrees. So you may have to move the sub to get it correct. A couple of inches can make a pretty big difference.
That said my speakers are rated to 40hz and I set my sub to 32hz to get the best blend when I use the sub. I say when 'I use the sub' is because when I stream from the internet music the lows are already exaggerated.