I'll throw my weapon of choice into the pile: the NEScaf switched capacitive audio filter.
From the description:
It is more flexible than previous filters for QRP rigs. It has two panel-mounted controls — a single center-detent potentiometer that sets the center frequency of the filter’s bandpass (default value settable between ~415Hz and ~1kHz), and a dual-ganged potentiometer that provides a continuously variable bandwidth control (from ~60Hz to ~1500Hz). This ability to smoothly vary the bandwidth is very useful.
QRP Club of New England used to offer this as a kit. I scratched together a Manhattan build and I'm tickled with it. Sound quality of the two cascaded Butterworth filters is fine and doesn't get weird until cranked down to 60Hz. I find tuning tuning the center frequency over to the answering station a lot easier and more intuitive than messing with a receiver offset.