Capacitance: Help me understand

There are few errors in kkirkpa's post. Yes, it has to do with the storage of electrical charge. But the idea of storing energy during loud passages and releasing energy during soft passages is a little off base. The signal from the pre-amp is constantly charging and disharging this capacitance. The energy is stored and released at the same frequency as the electrical signal, i.e. if fed with a 1000 cycle signal, the cable is being charged and discharged at this rate. The preamp sees this capacitance as part of the total load it must drive. Loudness really has nothing to do with it. It turns out that the higher frequencies are more affected by this than the lower. The higher frequency currents flow more easily through the capacitance than the low frequencies, so as Plato points out, you have less high frequency energy reaching the speakers as the capacitance increases. Some very wide bandwidth amplifiers like Spectral recommend using high capacity interconnects from the preamp to limit the very high frequencies getting into the amp.
The construction of the cable does determine this capacitance. There are three factors. Size, distance, and dielectric. Larger wires with more surface area and putting the wires closer together will increase the capacitance. Changing the dielectric (the material separating the wires like the teflon mentioned above) will also affect it. Air has the lowest dielectric constant and will give the least amount of capacitance.
That being said, I doubt that this is your problem. I would suggest experimenting with speaker placement as a way to affect your center image and soundstage. Start by moving them close together and toed in until you get a clearly defined center image with limited soundstage. Then move them apart and toe them out to expand the soundstage until your center image gets too diffuse. Somewhere in between is where you want to be. There will be a sweet spot where you get both. Of course, this also affects the frequency balance. It takes a lot of time a patience to get it right, but nobody said this was going to be easy.

Please let me clear up one small item in my post above. I knew that a vacuum was a better dielectric than air, but assumed that no vacuum interconnects existed. I was wrong. I saw an ad last night in Stereophile for "the worlds first vacuum interconnect." Who would have imagined?
I also don't think that "ANY interconnect that uses a foil shield is a high capacitance design." If enough space is left between the center conductor and the shield, it can be low capacitance.
I was curious about the Wireworlds, so I went to their website, but they don't post the electrical specs of their cables.
I agree with Sean that cables can affect the tonal balance of a system and that it is system dependent. I based my speaker placement theory on the description from Ignatz that he had "a too deep soundstsage with diminished volume from the center image."
Ignatz, I have some very low capacitance single ended homebrew cables that I would be happy to let you try. Send your address to me and I'll mail them to you. Also, what length do you need?