I was interested in this question and went looking on EBay
MM/MC phono preamp: converts phono signal to line level signal, low-noise integrated circuits and high-quality components are designed to ensure clear sound; with a MM/MC switch on the bottom, easy to select to match different kinds of turntables. LINE and 3.5mm headphone outputs: with stereo...
www.ebay.co.uk
This little unit would solve your problem although I have no idea about the resulting sound quality.
Just plug the phono lead from your T/T into the input and then an output lead can go to your amp - not to the phono input but a 'line level' input like the Aux. You can then plug your headphones into this and won't need your amp on if just listening through headphones.