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

^^^ Regarding correct height you can't generalize based on cabinet height..You MUST account for how far down the cabinet face the center of tweeter is..Ideally you want that about same as your ears,IMO about 36"high +-2"...My Harbeth M30.1's are 18" high BUT the tweeter sits almost 7" down the cabinet face,making 24" stands perfect for them..

@freediver and @yogiboy 
Thanks for the advice. You own speakers deserving of respect and you clearly have thought about this a lot. I will do careful measuring and will heed the advice not to get too tall a stand, nor one that rings like a church bell. 

For positioning of 2-way metal dome type tweeter applications, I sometimes prefer the tweeter just slightly above the ear and next driver down midrange/woofer just below it. Yes, centered between the two, yet the difference can be distinguishable on some speakers, particularly with midrange frequency and sometimes a tad more fullness achieved.

May not apply in @hilde45 ’s lower ceiling situation , but it’s easy to try.

In some applications, simply found getting some speakers up off the floor a bit more than traditional preferences can render some mediocre sounding speakers into something more, just by lifting them "up" a tad, fwiw and ymmv.

I just ordered some Skylan stands made in Canada. 

http://www.skylanstands.com/

@djones  I’m interested in your opinions. Didn’t you have a stand already?