It's a shame this thread appears to have deteriorated so rapidly as the OP asks a very good question. Hopefully we may be able to provide a general concensus if not a definitive answer.
On the face of it if you already own a pair of satellite type speakers adding a simple sub woofer appears to give you a full range system on the cheap. A box with a 10" speaker can be made for little money. But you do need to add extra for suitable a connection method, crossovers, amps, convienient controls and the like. You also need a lot of space or easy going neighbours. Probably some kind of room treatment too or all that bass rattling around is going to make your entire system sound boomy and muddy.
On the other hand . Your mains are your mains. 90% of the useful information is going to come from the 4"-6" midrange speaker. The very top and bottom of the audio range are the icing on the cake for those who can afford it and have the space.
Since the genuiely good stuff starts at around $400 pair (ADAM F5, Mackie MR5, Yamaha HS5, ADAM A3X etc.) it makes the most sense to first spend as much as possible on a quality set of active speakers going down to at least 60Hz (4" drivers or greater) before adding a sub.
That's how I see it anyway but it's an open question and I'd like to hear other opinions and why they are held?
To the OP. Speaker designers cannot beat the laws of physics however ingenious they may be. If you cannot face having a box big enough to hold at least a 4" ported design on your desk there is a limit t the quality of sound you can obtain. A sub will give you more boom and gravtas but it will not make your satellites hi fi. Consider a set of 5" cones on a pair of stands behind your desk.