This one can be an affordable ticket to STAX club. Thankfully found this high voltage OpAmp from APEX TECHNOLOGIES.
You will need to add own input circuit, which also does regulate volume and phase shifting (180°, each channel outer stators).
Second schematic is standalone, Lundahl transformer does the job, you can choose other types too. In this schematic a four row volume potentiometer is needed (ALPS). Drawback is limited output, only 142Vrms is possible. But should be plenty for normal volume listening (Stax sensitivities range ~100V/100dB).
These OpAmps on ebay are about ~50$/pc; you will need four of them (4 Stax stators). Still way cheaper than original Stax amp unit.