Can you suggest specific passive volume controls for me to research?
The C-core ("Reference C-Core) and double C-core ("Signature") TVCs by
Promitheus Audio are outstanding. Note that the Signature unit is not yet advertised on the web page, but it is available to order.
These Promitheus TVCs are very well implemented pure dual-mono (separate chassis for each channel) designs made with large transformers that are wound in house at the manufacturer's facility in Malaysia. The units are made to order and typically take about four to six weeks to be built after the order is placed. The TVCs are shipped by air into the U.S. with only two to three days in transit. I don't know whether there is any return policy, but you can write to the principal, Nicholas Chua, and ask.
When considering a TVC for your system, you will want to look at the input sensitivity of your amp to ensure that it is compatible with a TVC. Generally, you are looking for input sensitivity of 1.0V or less and input impedance of at least 50kOms.