Can't you just adjust the integrated amp's output to match whatever solid state is on bottom, then introduce a preamp (preferably passive) upstream of both amps to control the output from your source level?
Otherwise you could consider the adjustable EVS attenuators for the amp(s), see www.tweakaudio.com.
Otherwise you could consider the adjustable EVS attenuators for the amp(s), see www.tweakaudio.com.