The link is long dead, and I can't find the article among my old files. But Shawn Micheal's referenced post reminded me of what it was. Very very simple - just a B7 15K:500 transformer, followed by a 500 ohm potentiometer. It's only useful if you have way too much gain, but many systems do have way too much gain. Running the volume control at a low impedance of 500 ohms gives it much more drive. Many observers have found low-impedance level controls to sound better, though except for the drive current available there are no really plausible theories as to why.