You are not including the later machines cast in aluminium alloy, but anyway - in my experience most of the rigidity problems come from the carriage and cross slide, mainly if the bearings are not nipped up when taking a cut. The flexibility of the bars, both main and cross slide, probably contributes, but it's not so obvious. I don't think the cross slide bars contribute much due to their flexibility (compared to a nice big dovetail slide) as they are too short to move much. There it will be slack in the bearings. It is possible to nip them up, but who remembers to reach for the Allen key to do that? Well, I do, for one. The main 12 mm bars are more of a problem, and there have been schemes to use bigger bars, but it's not an easy thing to do. On the PC/Basic the bars are 20 mm diameter, which is much stiffer (about 4.6 times stiffer - a 15 mm? bar would be twice as stiff), but that was designed many years later when the constraints of post war Austria had disappeared. I'm still convinced the DB design was based on the availability of standard lengths of precision ground silver steel (drill rod), or something very similar. Any flexibility in the bed would be helped by bolting it down to a rigid base. Even a big chunk of wood would help.
