Differs from person to person, and system to system, but unless you are willing to step up to the plate big time for a good sub, cross over, and extra amp, my opinion is that you are almost always better off with a better set of speakers, than with adding a sub.
The theory behind adding a sub is good, but in practical terms they are difficult to get sounding "just right" without having to monkey with them all the time. They can be difficult to integrate, and often leave you with a sonic hump or pothole right around the crossover frequency.
Make absolutely sure to try before you buy if possible. Rel, Talon, and Aerial (just to name a few) all make good subwoofers, but you would be wise to look into a Vandersteen 2W or 2Wq to begin with. They are great bang for the buck, and are relatively easy to obtain and resell. You might only lose the cost of shipping for that experiment, and it might be worth a try just to satisfy your curiosity.
The theory behind adding a sub is good, but in practical terms they are difficult to get sounding "just right" without having to monkey with them all the time. They can be difficult to integrate, and often leave you with a sonic hump or pothole right around the crossover frequency.
Make absolutely sure to try before you buy if possible. Rel, Talon, and Aerial (just to name a few) all make good subwoofers, but you would be wise to look into a Vandersteen 2W or 2Wq to begin with. They are great bang for the buck, and are relatively easy to obtain and resell. You might only lose the cost of shipping for that experiment, and it might be worth a try just to satisfy your curiosity.