I think if one wanted to use this for headphones, the best thing would be to attach a headphone adaptor to the preamp out.
Something like this: https://www.zynsonix.com/Deluxe-Speaker-Amp-to-Headphone-Converter-Box_p_54.html
Something like this: https://www.zynsonix.com/Deluxe-Speaker-Amp-to-Headphone-Converter-Box_p_54.html