Congratulations on the AT33Sa! My last cartridge purchase was its mono sister, the AT33MONO (50th anniversary edition). It also sounded thin in the beginning, and then the body came out. Went through the biggest transformation during the first 25 hours, but kept steadily improving for hundreds of hours.
The best thing with it is that it can keep up with component changes extremely well. Upgrading phono stage, turntable - and it will keep up with it at an astonishing degree, given their relatively modest prices.
Have fun! ;