Reply from HomeThtrLA - I have spoken with a tech type at Belden about these issues, and I think I understand them fairly well.
First, in most cases, quad shielding is not necessary. I built a store about five miles (as close as you can get due to mountains) from all of Los Angeles's TV transmitting towers about six years ago, with a TV antenna and a distribution system sending signals out to about 450 televisions. The trunk runs were all used standard aluminum foil and braid, solid copper core RG-6, with similarly built RG-59 for the short runs between TVs. There was NO GHOSTING. As far as I can tell (and this makes the Belden guy chuckle), the one reason for RG-6 Quad is so that an installer who looks at a system I installed cannot say to the customer, "wow, gee, I wonder why these guys did not bother to install RG-6 Quad! It has more shielding so it is better!"
Second, TV antenna and standard channel cable signals do indeed travel on the surface of the cable (or to be more exact, between the surface of the inner conductor and the shield), so there is no difference for signals in this frequency range between "copper core" and "copper-clad" steel, as they are called. The copper-clad steel is stronger and does not stretch as easily as copper core, so it is a better choice for cable companies where a bit of stretch might accidentally occur.
The cable companies use RG-6, usually copper clad, with an added nonconductor, an 18 gauge steel strand called a "messenger," when they actually intend to drape the wire from one place to another. That wire is commonly used for the drop from the pole to your house. The added wire supports the cable and further limits its stretching.
However -- copper-clad steel is not as good for low frequencies such as DC, audio, and even normal video to some extent. The reasons are 1. the lower frequencies do not travel on the skin, so some of the signal goes through copper and some through steel, and of course the signal can just wander back and forth from one material to the other as it goes; 2. steel has a higher resistance than copper, so frequencies not going on the skin will be attenuated more, especially as the lower frequencies use the cable more as a wire than as a transmission line with a characteristic impedance. This actually limits the length of cable you can use for a DSS signal because DSS receivers send DC and 22 kHz up the wire, and a piece of copper-clad will look like a longer piece of copper-core; 3. Steel has a different (slower, I think) velocity of propagation, so the signals going down the copper will arrive at a slightly different time than signals going down the steel. This effect will be very minor, because the time difference is so short for any length of wire that, for the effect to be noticed, the actual frequency going on the cable would probably have to be so high that the signal would travel on the skin, so the effect would not be noticed anyway. 4. Copper braid has a lower resistance than aluminum braid (and foil), so it is the material of choice where signal levels cannot be attenuated without problems, such as CCTV...also the copper was the original technology that was soldered onto RCA connectors or crimped into BNCs before we started getting serious about quick-crimp F-connector styles.
Your best choice for the subwoofer would indeed be C. Sub frequencies definitely are not affected by the skin effect. In fact, I did some kind of calculation a few years ago and determined that, in the audio range, skin effect contributes to a tiny percentage of signal flow in the audio range -- the skin effect portion of the signal was something over 100 dB below the non-skin effect portion of the signal. And this was at 20 kHz.
