Surge protectors are for protecting against over-voltage events (lightning, etc.). We are talking about leakage current, i.e. electrical power being dissipated through the chassis. Your chassis is electrically coupled to mains via capacitance; this is either by poor design (parasitic capacitance), degradation of power-coupling capacitors (direct capacitave coupling), or a fault with your electrical grounding. Or any combination thereof.
The safety mechanisms against leakage current include: fuses, and ground fault circuit interrupter (GFCI), aka residual current device (RCD). Fuses protect against high-current faults and are more about preventing fires than preventing you from getting shocked. GFCI/RCD somewhat protect you from leakage current but are typically only found in bathrooms and kitchens in the USA. Some countries have whole-house RCD.
If the voltage on your chassis is high enough to feel, then it is high enough to damage sensitive electrical components. But the same could be said for the static electricity from your socks. In truth these voltages will be attenuated greatly by the design of the synth circuits and you are much more likely to get unwanted humming rather than any damage.
Inspection of your outlets is critical. I would also suggest removing or replacing your interface. Don’t worry about the health of your synths until you have taken measures to ensure your own welfare.
Some reading on the subject:
I’m Shocked: Why am I getting a Tingle?
Handbook for Sound Engineers: Leakage Current
Tingle says, “Stay safe; get an electrician! Koo-loo lim-pah!”
