Stock PS is $1K ($750 used or new if you look).Then one can step it up to PCX or Cullen mods for another $750.Won't touch EAR 324 (not sure if that has volume).Steelhead is the best at $4k used and you ge one line level input but you gotta pay.You can broaden your choices if you add TVC or Palacette volume controller.But remember you are going to be depending on what might not be best op-amp gain control so not having a pre might not sound as good and passive might not deliver bass or dynamics you might want.But depends on unit of course.
Chazz
Chazz