They are already running at half the frequency of the S1 (196 MHz), probably right at the bottom end of the voltage range for the chips. I doubt that there is much more room for significant undervolting.

In the config file, there is a lower setting for 193 Mhz. If I got an S2 with a weak power supply I'd probably try this setting first before upgrading the power supply. 1 TH/s makes so little these days that a good $300+ PSU replacement could blow your ROI right out of the water.