Bi-wiring with different cables for HF and LF?

I've tried it. Unless you really need some type of "band aid" fix for a problem, its usually a really bad idea. If you have really revealing components, you should easily be able to hear 2 different cables.

"But since plain stranded cable has somewhat fallen out of favor, I wondered if I might be able to improve the definition and articulation of my bass with a different cable for the LF run."

Who cares. If they work for you, use them.

Well, that's the thing. I haven't tried out that many cables for that LF run, so how would I know if another cable might be significantly better or not? That's why I posed the question. I am looking to maximize my bass impact and heft.

Richard Vandersteen also says its a bad idea.

I think you are asking too much of your cables. Even though cables can sound different from each other, you have to remember they're still passive components and can only do so much. A well designed cable should give you excellent sound in all areas. If you're not happy with your bass, a much better solution would be to focus on your active components. Upgrading your source, preamp or amp is more likely to fix your problem.

"Richard Vandersteen also says its a bad idea."

He's right. I was using Vandersteen speakers when I experimented with using different cables on the top and bottom. Its kind of hard to explain, but the system just sounded wrong. You really have to hear it for yourself.