The Sweetest Whispers were a shunt based attenuator built on plastic Lorlin switches back 15+ years ago for the Foreplay I/II/III. Due to the low service life of those switches and the difficulty of getting the sweet spot with various source output levels, that design was discontinued. We replaced this with a 36 step fine/coarse attenuator using open wafer Electro-switch switches which offer vastly longer service life, though they cost a heck of a lot more.
Having said all of that, the answer is that yes, you would need to purchase the entire first upgrade in order to obtain the upgrade parts for the 300B active loads.
Depending on which of the Khozmo relay attenuators you have, a custom chassis may also be a good idea.
