Music Composers Unite!
I hate to disagree with the other poster, but at minimum you need to have either:
A decent set of near field monitors that are full range (20hz to 20Khz), or
A decent set of near field monitors that are not full range (most with low frequency transducers less than 8" will not reach 20hz, and roll off at about 50 to 100hz) that have a matched sub.
A sub is a misunderstood commodity for the most part. You have the boom box subs like you hear in cars that sound like they are about to blow the windows out (we do not want this), then you have real subs that are designed to extend the frequency of the satellites down to the low range of the human norm. Hearing these frequencies in the mix is essential as a lot of harmonic distortion happens there, as well as a lot of external noise, like mic rumble and plosions.
A lot of music happens down there as well. You can tune a bass drum to have a fundamental at 45hz, which, without the sub, you would fail hear the fundamental, and only hear the harmonic overtones. You cannot mix properly this way.
Depending on budget, I would consider either KRK (mid price) or Adams (higher priced) but neither quite go to 20hz except in the high priced range.
What is critical about a sub is placement. Low frequency transducers suffer more from boundary effect (the tendency for sound waves to collect at walls and either amplify the frequency (constructive interference) or cause it to cancel out and cause dead zones (destructive interference). Bass below 50 hz is considered omnidirectional (IE our ear has difficulty locating the source), so there is some flexibility in placement to reduce the effect.
I use a sub in my monitor setup check it here
The first thing I did when installing was, get a favourite CD track that I know very well and attenuate the sub to around -4db to -6db until the crossover was transparent in the mix. Then get some other tracks of all sorts of music and just listen until I was satisfied of a balanced sound throughout my hearings frequency range. That's where Chris's point about training ears comes to the fore.
Now it may be fine in a cinema when a technician adjusts the bass up a little to get your seat rumbling but that can only be done if the original recording was truly balanced.