Thorens TD124 “Mk1”/Mk2 plinth

Yes, it might be more of a theoretical, rather than a practical, issue.  My dealer puts very expensive cartridges, like Platinum Signature Rosewood Koetsu cartridges on modern tonearms (typically Ortofon, Triplanar, or Audio Noe) on idler tables like the Thorens 124 and Garrard 301 and 401 and Lenco tables.  Of these tables, he likes the Thorens 124 the most.  

Great choice, I wish I had kept my TD124, it produced the best bass I ever had (with V15Vxmr beryllium cantilever tracking at 1.0gm on SME 3009) but that wonderfully machined bearing is very susceptible to vertical transmission, my floors were too springy, so think about it’s location, perhaps a wall shelf.

It's metal frame is solid, you do not need to stiffen it, you simply need to isolate it from any vibration getting to it.

Larry, you fail to specify whether the TD124 that causes no issues with the various cartridges you name is a mk1 or mk2 type. One sensitive way to check for magnetic attraction is to set VTF with a digital gauge and then, with the gauge still in place, gently raise the headshell by a mm or 2, and see whether the gauge goes negative briefly in the process.

I use Decca cartridges which incorporate relatively large magnets with my MKI TD124. I have no issue with the original iron sub platter EXCEPT this: I make tracking weight adjustment for these cartridges with an LP vinyl placed on the platter mat. The mat incorporates metal or other material interwoven (Auditorium 23), however the magnetic pull of the platter still affects downforce on the cartridge if downforce reading is taken with only the mat in place. Placement of an LP under the stylus to then check tracking weight, for my system, is the only way to obtain a more or less accurate readout. YMMV.