**Which tonearms will make the greatest improvement for the least money? And which tonearms are great matches with the turntable?**
The Alphason HR100S and the Zeta were bolt in replacements for the Ittock. If you can find one of these, it should be < $1K. The HR100 is S shaped and featured a one piece titanium armtube with integrated headshell. Nice arm, highly detailed, but you might need a bit of damping (internal, wrap, or fluid) with some carts. Mass is 11/12g.
The Zeta is a heavier arm - straight aluminum one piece with big bold bass, and sweet top. Mass is 16g. With an LP12 it's critical to have the suspension adjusted correctly. Knowing how to do this could determine your satisfaction level with the table. Both arms have height adjustment at the base with 2 allen wrenches and have high quality conventional bearings.
Dynavector 10X4 ?? Be wary - This is ancient and not worth the effort. Apparently Dynavector farmed out production and many were released with the channels labeled out of phase. It's not worth retip money anyhow. The 20X H should be much better. You might want to consider a vintage MM, many of which are better IMO than almost any HOMC.
Good luck with the table.