Adding a headphone amp to a BAT preamp

I've been fooling around some with my headphone amp, and would like opinions on how to best use it with my BAT VK30 preamp. I have a phono stage, tuner, CDP, and cassette deck plugged into the various preamp inputs. I have balanced interconnect running from the main balanced preamp outputs to the balanced inputs on my amp. Since the cassette deck uses the tape output from the preamp, I tried plugging my Grado headphone amplifier into the unused RCA main pre-out. It works, but so do my speakers. I'd like to be able to throw my Senns on my head and effectively mute the preamp output to the speakers, controlling the volume to my headphones via the gain knob on the Grado headphone amp. I'm pretty sure there's a simple solution, I called BAT but they never returned my call. Can anyone help? Thanks, Jeff