With a gain of 19dB, the output must be 13.4 volts, which is 22 watts into 8 ohms. You'll get 102dB peak sound if the speaker is 88.6dB sensitivity, and that would be a good combination. Foreplay has approximately unity gain with the volume set in the middle, and I'm assuming the Red Book standard of 2vrms pull scale from a CD player. If your speaker is significantly more sensitive, you'll probably want to reduce the gain somewhere to maintain a reasonable setting on the Foreplay.
For more details, see my white paper on the Bottlehead Community page.