Kip's problems may not have been 100% speaker related. You could spend thousands on speakers and still never get a mix to translate, not even once. It's all in the acoustics of the room. If you're simply mixing to get a sound or rough demo, use a woofer and the most decent speakers you got available to you. I reccomend this to get you used to mixing. But if you wanna get serious about mixing, get some decent active monitors for around $200 for the pair. Use the subwoofer ONLY to support the low end, not accentuate it. This is tricky to do.
The rest is your room. Save the money you wanna spend on super expensive speakers and put it into treating the room. Do research on setting up the room properly. A nice place to start is deaden the wall behind ur monitors and diffuse the wall behind you. If you're area/room is less than ideal, simply do ur best to absord and diffuse as much as possible. DO NOT deaden the room completely. Rather, try to CONTROL the room with well placed absorption, diffusors and bass traps. Corner traps are good too. The main goal should be to simply try and stop the standing wave patterns which are most likely causing you some cancellation or accentuation: perhaps, in ur case, ur acoustics may be accentuating certain frequencies so when ur mix sounds full in your working area, grat. But when you listen to it elsewhere, it sounds empty or hollow or not as full. The speakers ur using may be coloring the sound, as well, hence the need for flat response, decent monitors, which I see you said you are saving up for.
I see no problem using a subwoofer to support the low end but you have to know what you're doing AND treat the room first. If you simply want the woofer to strngthen the low end, that's silly, becuz the low end may be getting cancelled out at the point in the room where you listen or enhanced fraudulently, so when you listen elswhere to the mix, it lacks bass defention or low end altogether.
Worry more about addressing the acoustic problems in a room BEFORE montior quality and ur mixes will translate EVERY time. For some good help on acoustic treatment, I suggest "Hal Leonard's Recording Method- Book 2: Instrument & Vocal Recording" for a good introduction to the whole acoustic concept.