This is all really fascinating, thanks for sharing! So I made an 8 x 4 machine frame from I beam profile filled with concrete mix (vid of it being made, concrete mix design details etc & it cutting: https://youtu.be/cqYJS27aC4w ) and wish I had made more video recordings of how the concrete changed the vibration characteristics. I actually used a vibration monitor on my smartphone, and had the data, but that phone got hit by a freak flying phone seeking branch when we cut a dead ash tree down :(

I suspect you have seen it, but I haven't seen mention of it here yet: Bamberg's "Principles of Rapid Machine Design" In it he describes a similar concrete-filled tube, but he adds constrained layer damping... Worth a look if you haven't seen it - page 96 on.