Adding a sub woofer?

One subwoofer can be very frustrating to an astute listener. Best to get at least two to start. Then they are matching. If you can't afford two save longer. 
If you use an appropriate crossover they will not color the midrange at all but interestingly give you more sense of depth in the midrange. There is more air in the room. It is an effect that most people notice. Turn off the subs and everything goes flat. But, the crossover is critical and the only ones that offer total control of frequency and slope are digital. People who are married to total analog miss out on this. JL Audio makes a decent analog crossover. 

@ieales, absolutely. I think top model JL Audio subs may be the best available and their price reflects that. To my knowledge nobody makes a stand alone digital bass management system. The most sophisticated versions come with full room control units/preamplifiers. I must say however that the current units are easier to set up than the TacT units were. Almost any audiophile should be able to get decent results. The hardest part is setting up the microphone in the right place. The down side is the additional expense.
Having toiled with subwoofers since 1978 I can say unequivocally that you can not get the full benefit of subwoofers without a complete 2 way crossover,  be it analog or preferable digital. For those unfamiliar with this, the crossover goes between the preamplifier and the amplifiers. All connections are line level. Using speaker level connections drops the impedance to your amplifier decreasing it's damping. It is not acceptable.
Your money would be best spent upgrading your system before getting subs.