Maybe all you need is something like the EVS or Endler attenuators that connect to the inputs of your amps and allow you to attenuate the signal coming in from your preamp. Both of these products are transparent and work quite well.
If you want to try a full fledged passive preamp then I would recommend a transformer volume control (TVC) such as the Prometheus or some of the Stevens & Billings based models that are available.
If you want to try a full fledged passive preamp then I would recommend a transformer volume control (TVC) such as the Prometheus or some of the Stevens & Billings based models that are available.