Hi Alex,
26dB gain is 20X amplifier. The power supply for DRV595EVM is 26V, there is no boost circuit inside the power driver. So I don't think the output amplitude could reach 100V. Could you please double-check your setting? You can find more info about DRV595EVM in this document: http://www.ti.com/lit/ug/slau479/slau479.pdf.
Best regards,
Shawn Zheng