That's not really what those resistors do, and you'd have to use a very high value to accomplish that and it would depend on the input impedance of the amp that's too loud. This would add to the output impedance of the preamp, which isn't ideal. The Moreplay has very, very low gain, so I would wonder what amp and speakers are causing the problem? You could probably get away with the Rothwell attenuators at the input of the second amp, but I would suggest posting a more complete picture of all of the components (speakers included) being used here.