If a speaker stand is mass-loading, how much does it matter what they're made of?

It matters, because everything has its own peculiar set of vibrations, resonances, sounds it makes- whatever you want to call it they are different from one to another. 

Easiest way to get this is to hit em and listen. A nice sharp knock that has no real tone or character (can't tell if you hit steel, plastic or wood, doesn't really sound like anything identifiable) and dies real fast is ideal. 

But really, even if you get one that isn't all that great you can still be fine it just means a little more work selecting the right cone or material to use between the speaker and the rack. Use springs for example and the speaker will float and then what the stand is made of will matter much less as long as its massive.

Its been a long time since I was in the market for a stand. But even back then I was surprised how much better an inexpensive professionally designed stand was than the jury rigged stand I was using. Which basically was putting the speaker on top of something the right height. Not really a stand. 

So the happy ending of this long story is you will probably be fine to just get the one you like the looks of, that looks well made. Whatever it is, you will almost certainly be able to make it even better with something as simple as blue-tack, cones, or springs.

millercarbon -- thanks, that’s helpful. I’m trying to decide what to buy, so I am don’t have stands to compare. I have MDF stands -- I know how they sound. I don't have other kinds of stands, including steel. But knowing how fill material could affect resonance might help with a choice, and reduce cost, possibly. I’ll be curious to hear what people have found. If you have a view on that, I welcome hearing it.

@hilde45
With the above in mind, any opinions on modestly priced stands? Perhaps 20" - 22" high?

(Again, I'm not in the market for expensive stands, right now.)

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Yes. Support your local metal shop.  Call and shop a few local metal shops/fabricators. Easy to build, two plates for top and bottom, metal posts, weld, add screw-in spike nut-serts or tap threads, paint, done. Super easy for average fabricators.

With people out of work in a pandemic, any reasonable shop still open can handle this. Thicker metal, weighty, custom height, can be lower price than buying cheap thin metal junk online and USA Made! :)