Wood stands that you put together are cheaper to pack and ship than welded metal stands that take up a lot of room on the truck. Plus, welded stands are more labor intensive and usually heavier, so there is additional cost there. So, the likely reason PSB uses mdf stands is cost.
As for sound differences, I dont know that you can attribute sound differences to the stand material, all other things being equal, which they never are.
As for sound differences, I dont know that you can attribute sound differences to the stand material, all other things being equal, which they never are.