You could use set of Rothwell attenuators. They work extremely well.
http://www.rothwellaudioproducts.co.uk/html/attenuators.html
"Although the attenuators can be used on the inputs of the pre-amp in exactly the same way as they would be used on an integrated amp (with the same benefits), alternatively they can be used between the pre-amp and the power amp to even greater effect. When used this way, not only is system gain reduced by 10dB, but the signal to noise ratio is improved by 10dB. This pushes the noise floor down enough to create an inky black silent background from which the music can emerge with better resolved fine detail and transparency".