Industrial grade CNC router (that can mill steel) with concrete

[tonyfoale]

You eloquently echo my own feelings regarding the lack of suitability of the T-slot aluminium extrusions for such projects. I have also used concrete in some machine projects, both to add mass and damping.
Great job. It is hard to get machine tool level degrees of stiffness on such large structures but you seem to have succeeded well.
Judging by the motor noise I guess (but may be wrong) that you are using conventional steppers. If so I would suggest looking at feedback or closed loop steppers. The difference is huge, I found this out when I once had a driver with the feedback switched off accidentally. Nowadays the price difference is small.