Card zero was recommended for stations running SCAs because it protected the SCA region from splatter better than a stock Optimod. I am not sure whether or not gaining added loudenss was the original goal of card zero, though that is certainly an extra benefit. Card zero can be removed by jumpering the safety clippers of card 8 and 9 back in, but if it's not broken, you should probably leave it in place.
Back in the late 1980s, a fledgling company that evolved to produce major audio processing gear, made a 53kHz brickwall filter that was inserted into the composite audio stream. It was intended to protect both the subcarriers and promote extra loudness by eliminating overshoots, though we never got ours to produce satisfactory results. It looked good on paper for several reasons, but the filter tended to ring with heavily processed audio, producing its own overshoots. Card zero did work nicely, however.
