In many ways a speaker that goes down smoothly to 40Hz is an ideal candidate for a subwoofer. It's much easier to crossover from the sub to main speaker if the main has reasonably deep bass response. It allows the sub to just work the deep bass without having to work up into the mid-bass region. However, most popular music doesn't have alot of content below 40Hz. A typical pop mix hypes the 80Hz region in lieu of deep bass, so you shouldn't expect to hear a subwoofer on all music. In fact, if the sub is correctly setup, you shouldn't hear it all that often. But when it hits -- you should feel it. None of the other changes you're considering will do what a good, well setup subwoofer can.
Great sound possible using a Subwoofer?
My speakers are flat to 38 hz but I feel I'm still missing impact, power and weight on most of the pop/rock CDs I listen to. I don't know if a high quality subwoofer would be a positive addition to my system or just add unrealistic bass and muddy up my midbass and midrange. If anyone has extensive experience with this dilemma please tell me what you think. (would better isolation of components, cables or power conditioner help the low end?)