If you know all the tricks you can set a sub up in a week and be close. If not, you could fiddle around for months and still not be entirely satisfied.
Phase is particularly important if you want adequate output and a tight clean sound that has that dynamic punch. Many times if you are using a single sub it is best to use only one channel of the output. Particularly if you are crossing over at frequencies lower than 50hz. No real separation down there anyway.
Placement to avoid nulls is critical too. Once setup you move toward and away from the sub. You'll find spots where volume is low and spots where it is high. If low, you are in the trough, you want the listening to be at the peak.
Some of the EQs are quite good at damping the peaks commonly caused by subs in smaller rooms and can be very helpful as well. Avoid using the the sub crossover between your preamp and amp, this generally reduces the purity of the signal going to your main amp. I usually try to use one set of outputs from the preamp for the power amp and a second set, if you have them, to feed the sub. That way there is no interaction electrically.
For those of you with high level inputs on your subs, this is the ultimate way to setup a powered subwoofer, though few realize it.
Using a set of speakers cables from your amp to your subwoofers high level input is actually the most accurate method for interfacing your sub, main speakers, and amplifier. Your sub is now receiving the signal from your amp, so it takes on the characteristics of your amp. Dynamics, phase, all much more like what the main speakers are seeing. Much better blend, try it, you'll be pleasantly surprised. I mean how can it sound like the amp when it's being fed by the preamp? It takes no power from the amp, the high level input on the sub is very high impedance, so it is really isn't seeing a great deal more signal that it would be from the preamp. However, the amplified signal is more dynamic, more vivid, and as you might guess, suits the nature of a powered sub very nicely.
